Preparation of high-performance Cu₂Se thermoelectric materials by the KCl flux method and research on thermoelectric transport performance

Abstract This study achieves a notable enhancement in the thermoelectric performance of copper selenide compounds exhibiting liquid-like characteristics via an innovative processing method. A KCl flux-assisted high-temperature melting and slow-cooling strategy was employed to fabricate nanolayered Cu 2 Se (KCl) x materials ( x = 0–3, denoted as S 0 –S 3 ). Systematic characterization reveals that the coexistence of α and β phases at room temperature creates favorable conditions for optimizing carrier transport. XPS analysis confirms the substitution of low-binding-energy Se 2− by high-binding-energy Cl − ions within the lattice, effectively suppressing copper ion migration and remarkably improving the material’s structural stability. Microstructural investigations demonstrate that all samples exhibit nanolayered stacking architectures abundant with edge dislocations. This multiscale defect architecture induces strong phonon scattering effects. Hall measurements indicate that the KCl flux-assisted processing facilitates the formation of highly ordered nanostructures, thereby enhancing carrier mobility and structural stability. Although the carrier concentration exhibits a slight decrease compared with the flux-free samples, the significant improvement in microstructural quality plays a crucial role in the synergistic optimization of electrical conductivity and the Seebeck coefficient. Notably, sample S 2 exhibited a considerable electrical conductivity, reaching approximately 1.0 × 10 5 S⋅m −1 at 300 K. More strikingly, the cooperative effect of high-density edge dislocations and dopant atoms elevates material entropy, enabling sample S 3 to attain an ultralow lattice thermal conductivity of 0.55 W⋅m −1 ⋅K −1 at 350 K. Through multi-mechanism coordination, sample S 2 achieved a high ZT value of 1.45 at 700 K, representing a 2.7-fold improvement compared with traditional synthesis methods. This work provides new insights into performance optimization of liquid-like thermoelectric materials through defect engineering and entropy manipulation.