Enhancement of thermoelectric properties and first-principles calculations of Ag / Ti co-substituted Ca₃Co₄O₉

Thermoelectric materials of Ca₃Co₄O₉ with excellent comprehensive performance have stagnated the research of thermoelectric conversion devices due to their low thermoelectric conversion efficiency. Ca_3-xAg_xCo_4-yTi_yO₉ was synthesized by double substitution at Ca and Co sites via a traditional solid-state method to change the microstructure and improve the thermoelectric properties in this paper. Ag and Ti co-substituted increase the porosity, reduce the concentration of Co⁴⁺ ions, and improve the Seebeck coefficient and power factor. The optimum substitute amount is x = 0.2, y = 0.2, and the Seebeck coefficient and power factor are 238.4 μV/K and 1481.9 μW/K²m. The first-principles calculation results show that the addition of Ti increases the covalent bond strength between Co–O atoms, and the addition of Ag narrows the gap, which is beneficial to the improvement of thermoelectric properties.