Defect Chemistry Engineering to Regulate the Excellent ZT ave Value of Nonstoichiometric Ca3–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 Ca3–x Co4O9 (0 ≤ x ≤ 0.06) by defect chemistry engineering to regulate the excellent ZT ave value. The Ca-active sites of Ca3–x Co4O9 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 Ca2.96Co4O9 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.