Defect Chemistry Engineering to Regulate the Excellent ZT_ave Value of Nonstoichiometric Ca_{3- x}Co₄O₉ (0 ≤ x ≤ 0.06) Ceramics

Cobalt-based oxides have attracted significant attention as p-type thermoelectric materials due to their wide operational temperature range. However, their low average figure of merit (ZT_ave) value has hindered service performance. A series of cation vacancies as Ca-active sites were introduced into Ca_3-xCo₄O₉ (0 ≤ x ≤ 0.06) by defect chemistry engineering to regulate the excellent ZT_ave value. The Ca-active sites of Ca_3-xCo₄O₉ ceramics induced lattice distortion and point defects, which were unequivocally confirmed by the iDPC-STEM results. The power factor from 0.18 mW/(m·K²) to 0.38 mW/(m·K²) and the electrical conductivity from 54.8 to 108.3 S/cm were achieved for the Ca_2.96Co₄O₉ sample. A notable ZT value of 0.32 was obtained at 1073 K. Furthermore, the high ZT_ave of approximately 0.166 reached within the temperature range of 323-1073 K, representing a 3.25 times improvement compared to pure Ca₃Co₄O₉ ceramics. This study highlights defect chemistry engineering as an effective strategy for optimizing the thermoelectric performance of nonstoichiometric Ca₃Co₄O₉ ceramics, offering promising prospects for the development of Ca₃Co₄O₉-based thermoelectric materials.