Multifunctional Phase Engineering Expands the Carrier Concentration Optimization Window for High‐Performance Mg 3 (Bi, Sb) 2 Thermoelectrics

ABSTRACT The optimization of thermoelectric materials is fundamentally limited by the narrow carrier concentration ( n H ) window, which limits n H as an effective degree of freedom for enhancing performance. Here, we demonstrate that the introduction of a multifunctional phase overcomes this inherent limitation in Mg 3 (Bi, Sb) 2 . Specifically, the work‐function difference between NbC and the Mg 3 (Bi, Sb) 2 matrix drives interfacial electron transfer, establishing an elevated n H regime. NbC‐induced grain growth suppresses grain‐boundary scattering, thereby improving carrier mobility and significantly boosting electrical conductivity beyond the conventional n H optimization window. Meanwhile, interfacial charge redistribution and magnetic interactions associated with NbC modulate energy‐dependent carrier scattering, increasing the scattering factor and enabling the Seebeck coefficient to be maintained or even enhanced under high n H . Consequently, the optimized composites achieve an outstanding average power factor of 3.1 mW m −1 K −2 over 300–573 K, along with a maximum zT of ∼1.3 at 523 K and an average zT of ∼1.1. Furthermore, the fabricated single‐leg device achieved a conversion efficiency of ∼5.5% under a temperature difference of 282 K. This work breaks the conventional paradigm that restricts thermoelectric performance optimizing solely within the optimal n H regime, highlighting the pivotal role of multifunctional phase engineering in unlocking new performance frontiers.