LAUSR

While possessing outstanding electrical properties, suppressing the lattice thermal conductivity (κlat) is of great significance for achieving excellent thermoelectric materials. Here, based on optimizing electrical transport by transforming monoclinic Cu2SnS3 into a cubic phase, Halogen atoms are employed alloying to enhance anharmonicity, effectively suppressing phonon propagation in high‐symmetry materials, thereby reducing κlat while maintaining excellent electrical transport properties. An alloying study of CuX (X = Cl, Br, I) with Cu2SnS3 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 Cu2Sn0.94S3‐20 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 Cu2SnS3 system. These findings highlight the potential of harnessing halogen‐induced anharmonic effects to facilitate high‐performance thermoelectric applications, underscoring the viability of Cu2SnS3 as a candidate for sustainable energy solutions.