Abstract: Synergizing high performance and long-term stability in thermoelectric materials remains a formidable challenge, particularly for eco-friendly Cu-S compounds, which have outstanding performance but are plagued by thermal degradation at elevated temperatures and copper ion migration under electric fields. Herein, a promising in-situ heterostructure (Cu_2-xS-CuInS₂) strategy induced by isoatomic In substitution in Cu_1.93S was demonstrated to unlock the threefold balance among thermoelectric performance, thermal stability, and electrical stability. This strategy tunes the Cu vacancy content in the primary Cu_2-xS phase and the fraction of the secondary CuInS₂ phase, thereby suppressing carrier scattering while preserving ultralow lattice thermal conductivity and a high power factor. The optimized Cu_1.89In_0.04S achieves a peak zT of 1.57 at 873 K and an average zT of 1.31 over 723-873 K, ranking among the highest values within binary Cu-S thermoelectrics. Crucially, the heterostructure strategy imparts exceptional durability, as evidenced by negligible degradation under thermal cycling and remarkable resistance to current-induced failure, wherein ionic hysteresis at heterointerfaces suppresses Cu migration. Overall, these results underscore heterostructure engineering as a compelling route to transcend the intrinsic limitations of Cu-S compounds, charting a pathway toward next-generation of thermoelectric materials that couple high performance with long-term reliability.

Key words: Cu_2-xS, CuInS₂, Heterostructure, Thermoelectric, Thermal stability, Electrical stability