Harnessing Halogen‐Induced Anharmonic Effect to Achieve Low Lattice Thermal Conductivity in High‐Symmetry Cu ₂ SnS ₃ for High‐Performance Thermoelectric Applications

Abstract While possessing outstanding electrical properties, suppressing the lattice thermal conductivity ( κ _lat ) is of great significance for achieving excellent thermoelectric materials. Here, based on optimizing electrical transport by transforming monoclinic Cu ₂ SnS ₃ into a cubic phase, Halogen atoms are employed alloying to enhance anharmonicity, effectively suppressing phonon propagation in high‐symmetry materials, thereby reducing κ _lat while maintaining excellent electrical transport properties. An alloying study of CuX (X = Cl, Br, I) with Cu ₂ SnS ₃ is conducted and the correlation between anharmonicity and the ionic character in chemical bonds is examined. As symmetry increases, the power factor (PF) of the samples rises dramatically from 0.96 to 7.8 µW cm ⁻¹ K ⁻² , further increasing to 12.77 µW cm ⁻¹ K ⁻² with the introduction of Sn vacancies. A comprehensive analysis of band structure, anharmonicity, and lattice distortion reveals that the CuBr‐alloyed sample exhibits significantly higher performance compared to the other variations. Ultimately, the optimized Cu ₂ Sn _0.94 S ₃ ‐20 mol% CuBr reaches a peak ZT of 1.17 at 773 K and achieves one of the highest average ZT of 0.70 within the Cu ₂ SnS ₃ system. These findings highlight the potential of harnessing halogen‐induced anharmonic effects to facilitate high‐performance thermoelectric applications, underscoring the viability of Cu ₂ SnS ₃ as a candidate for sustainable energy solutions.