Half-Heusler (HH) thermoelectric materials exhibit excellent electronic transport properties but suffer from intrinsically high lattice thermal conductivity, which limits their thermoelectric performance. To address this challenge, double half-Heusler (DHH) compounds with 18 valence electrons have recently been proposed. However, the disordered nature of DHH phases often degrades their electronic transport properties, hindering the achievement of high thermoelectric conversion efficiency. In this work, we design a new n-type Ti₂CoNiSb₂ DHH compound with 18.5 valence electrons by combining TiCoSb and TiNiSb half-Heuslers, intentionally breaking the conventional valence balance. Furthermore, the effects of Cu and Bi doping at the Ni and Sb sites, respectively, are systematically investigated. Cu doping effectively enhances phonon scattering through point defects, while Bi doping significantly improves the weighted mobility. When Cu and Bi are co-doped, phonon scattering is further strengthened, particularly at high temperatures, and the weighted mobility is simultaneously increased. As a result, a peak figure of merit (zT) of ∼0.82 is achieved at 973 K in Ti₂CoNi_0.9Cu_0.1(Sb_0.925Bi_0.075)₂, nearly four times higher than that of the pristine Ti₂CoNiSb₂ (zT ∼0.22). This work highlights the effectiveness of co-doping strategies that simultaneously optimize thermal and electronic transport properties in DHH thermoelectric systems.

Bi₂Te₃-based materials have drawn much attention from the thermoelectric community due to their excellent thermoelectric performance near room temperature. However, the stability of existing n-type Bi₂(Te,Se)₃ materials is still low due to the evaporation energy of Se (37.70 kJ mol⁻¹) being much lower than that of Te (52.55 kJ mol⁻¹). The evaporated Se from the material causes problems in interconnects of the module while degrading the efficiency. Here, we have developed a new approach for the high-performance and stable n-type Se-free Bi₂Te₃-based materials by maximizing the electronic transport while suppressing the phonon transport, at the same time. Spontaneously generated FeTe₂ nanoinclusions within the matrix during the melt-spinning and subsequent spark plasma sintering is the key to simultaneous engineering of the power factor and lattice thermal conductivity. The nanoinclusions change the fermi level of the matrix while intensifying the phonon scattering via nanoparticles. With a fine-tuning of the fermi level with Cu doping in the n-type Bi₂Te₃–0.02FeTe₂, a high power factor of ~41 × 10⁻⁴ Wm⁻¹ K⁻² with an average zT of 1.01 at the temperature range 300–470 K are achieved, which are comparable to those obtained in n-type Bi2(Te,Se)₃ materials. The proposed approach enables the fabrication of high-performance n-type Bi₂Te₃-based materials without having to include volatile Se element, which guarantees the stability of the material. Consequently, widespread application of thermoelectric devices utilizing the n-type Bi₂Te₃-based materials will become possible.