Electronic Structure and Thermoelectric Power Factor of Na_xCoO_2 from First-Principles Calculation

To investigate the relationship between the electronic structure and the power factor of Na_xCoO_2 (x=0.3, 0.5 and 1.0), the first-principles calculation is conducted by using density functional theory and the semi-classical Boltzmann theory. Our results suggest that with the decreasing Na content, a transition from semiconductor to semimetal is observed. Na_0.3CoO_2 possesses a higher electrical conductivity at 1000 K due to its increased density of states near the Fermi energy level. However, an optimal Seebeck coefficient at 1000 K is obtained in Na_0.5CoO_2 because of its broadened band gap near the Fermi energy level. Consequently, a maximum power factor is realized in Na_0.5CoO_2. Thus our work provides a complete understanding of the relationship between the electronic structure and the thermoelectric power factor of Na_xCoO_2.