Nanotwin‐Engineered (Sb,Bi) ₂ Si ₂ Te ₆ for Robust and High‐Efficiency Layered Thermoelectrics

Layered chalcogenides, while efficiently suppressing phonon transport via weak van der Waals bonding, inherently suffer from poor mechanical robustness. Developing these materials that exhibit both high conversion efficiency and mechanical robustness is a critical challenge for their reliable device deployment. Here, we demonstrate a shear‐strain engineering strategy that utilizes dense nanotwins as a dual‐functional structural unit to solve this intrinsic limitation. Using (Sb,Bi) 2 Si 2 Te 6 as a model system, we introduce high‐density nanotwins via high‐energy ball milling. Mechanically, these nanotwin boundaries act as barriers to dislocation motions, doubling the compressive strength to 191 MPa and hardness to 0.8 GPa—levels comparable to mechanically robust cubic systems. Functionally, the nanotwins introduce broadband phonon scattering, driving the lattice thermal conductivity down to an ultralow 0.27 Wm −1 K −1 and enabling a peak ZT of 1.5 at 773 K. The practical viability of this synergistic strengthening is validated by a single‐leg thermoelectric device achieving a high conversion efficiency of 8% (Δ T = 473 K) with high structural integrity. This work establishes nanotwin engineering as a generalizable paradigm for transforming fragile layered crystals into robust, high‐efficiency thermoelectrics.