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Liquid‐like thermoelectric (TE) materials have the advantages of ultrahigh performance, low cost, and environment friendly, but their stability is greatly limited by the possible Cu/Ag deposition under a large current and/or temperature gradient. The pratical application based on liquid‐like TE materials requires both a high TE figure of merit (zT) for high energy conversion efficiency and large critical voltage for good stability, but they are very difficult to be simultaneously achieved in one material. In this work, both the zT and critical voltage are simultaneously optimized in Cu2Se via tailoring chemical compositions at multiple atomic positions, i.e., introducing Cu deficiency at the Cu‐sites to lower Cu ion chemical potential and alloying sulfur at the Se‐sites to reduce carrier concentrations. A maximum zT of 2.0 at 1000 K has been successfully achieved for Cu1.96Se0.8S0.2, about a 30% improvement over that for Cu2Se. More importantly, Cu1.96Se0.8S0.2 demonstrates a much higher critical voltage than Cu2Se, yielding a greatly enhanced service stability under the conditions with/without a temperature gradient. An Ni/Mo/Cu1.96Se0.8S0.2 TE unileg is successfully fabricated with a stable power output even after 400 thermal cycles between 473 and 873 K. This study greatly accelerates the real application of Cu2Se‐based liquid‐like materials. [ABSTRACT FROM AUTHOR]