Enhancing thermoelectric performance of Ca3Co4O9+δ oxide ceramics through lithium doping

Misfit-layered calcium cobaltite Ca3-xLixCo4O9+δ (x = 0, 0.1, 0.2 and 0.3) were prepared by the sol-gel route, and the resulting samples were systematically investigated in terms of their phase composition, microstructure, electrical and thermal transport properties. The experimental results revealed that all the samples consisted of the Ca3Co4O9+δ phase and traces of the Ca3Co2O6+δ secondary phase. The chemical composition of the polycrystalline samples was found to be close to the nominal composition, and a characteristic plate-like grain morphology with a high density of grain boundaries was observed. Lithium doping reduced thermal conductivity by 20% (0.81 W/(m.K)) at 300 K for x = 0.3) and yielded the highest thermopower (0.167 mV/K) with a power factor of 0.239 mW/(m⋅K2). An impressive figure of merit value of 0.07 was achieved at room temperature, which was further improved by about 25% compared to the undoped sample.