Enhancement of thermoelectric performance in Cd-doped Ca3Co4O9via spin entropy, defect chemistry and phonon scattering†
The misfit layered cobaltate Ca3Co4O9 has been widely investigated, together with other thermoelectric transition-metals oxides, because of its natural layered structure and its strong anisotropy in its thermoelectric properties. The thermoelectric performance of Ca3Co4O9 has been optimized over the past 10 years, but a method to strategically improve the thermoelectric properties is still lacking because of the complicated interplay among the misfit structural distortions, the strong spin entropy and the anisotropic charge transportation. In this work, by combining state-of-the-art X-ray absorption fine-structure spectroscopy (XAFS), X-ray photoelectron spectroscopy and thermoelectric transport property measurements, an approach is identified for enhancing the thermoelectric performance of Ca3Co4O9. The XAFS technique, by combining experiments and multiple scattering theory, reveals unambiguously that Cd occupies the Ca site rather than other sites. The direct effect of Cd replacing Ca is an incremental increase of the charge carrier concentration and also the distinctive charge redistribution, which induces a spin-entropy enhancement in the system by changing the net valence of Co, resulting in an enhancement of the electrical conductivity and Seebeck coefficient. Furthermore, the excess Cd also forms CdO nano-inclusions, which serve as phonon scattering centers and thus reduce the total thermal conductivity. Consequently, the figure of merit, ZT, has been enhanced remarkably from 0.15 to 0.35 at 1000 K, and this is attributed to the synergy of charge redistribution, spin entropy, defect chemistry and phonon scattering.