Thermoelectric Properties of Na x CoO2 and Prospects for Other Oxide Thermoelectrics

Abstract

We discuss the thermoelectric properties of Na x CoO 2 using the electronic structure, as determined in first principles calculations, and Boltzmann kinetic transport theory. The Fermi energy lies near the top of a manifold of Co t 2g bands. These t 2g bands are separated by a large gap from the higherlying e g states. Although the large crystal-field splitting implies substantial Co-O hybridization, the bands are narrow. Application of standard Boltzmann transport theory to such a narrow band structure yields high thermopowers in accord with experimental observations, even for high metallic carrier densities. The high thermopowers observed for Na x CoO 2 can therefore be explained by standard band theory and do not rely on low dimensionality or correlation effects specific to Co. We also present results for the cubic spinel structure ZnRh 2 O 4 . Like Na x CoO 2 , this compound has very narrow valence bands. We find that if it could be doped with mobile carriers, it would also have a high thermopower, comparable with that of Na x CoO 2 .

Spin-entropy contribution to thermopower in the[Ca2CoO3–t]0.62Hejtmánek1, Jirák2, Šebek3 2015Phys. Rev. B170100View full textAdd to dashboardCite

“…Very large Seebeck coefficient is possible only when the respective density of states or electron group velocities are very unlike below and above the chemical potential, which is indeed assumed to occur in the present cobaltates due to a particular "pudding mold" character of the a 1g subband in which Fermi level is located [9]. The numerical calculations performed within the generalized Mott formula (1) with a use of the ab-initio determined electronic structures for Na 0.7 CoO 2 provide, indeed, a large positive Seebeck coefficient with temperature dependence close to what is actually observed [10].…”

Data-Driven Review of Thermoelectric Materials: Performance and Resource Considerations

“…5,30 Likewise, even though Na x CoO 2 is metallic, with polycrystalline samples having r 300 K ⇡ 0.002 W cm, the correlated behavior of electrons and spin contribution to thermopower-or, arguably, the unique band structure-lead to a remarkably high Seebeck coefficient. 31,32 Looking at the ensemble of data (figure 4) also provides insight about particular families of materials. For example, we find that optimizing non-metallic moderate-performance materials is unlikely to lead to dramatic gains in performance, as the electrical resistivity is too high.…”

Preparation of Well-Tiled γ-Na x Co2O4 by a Novel and Simple Sodium Alginate Gel Method and Its Electrical Properties

“…[1][2][3][4] Since Terasaki et al found the high thermopower and conductivity of NaCo 2 O 4 single crystals at 300 K in 1997, 5 the sodium cobalt oxides Na x Co 2 O 4 have been widely studied for thermoelectric applications. [6][7][8][9][10][11] It was verified that the thermoelectric properties of a Na x Co 2 O 4 polycrystalline sintered body could be improved by obtaining high crystallographic orientation using special technology such as hot-pressing, 6 cold high-pressure, 7 spark plasma sintering, 8 and reactive templated grain growth method. 9 Biomacromolecules such as lipids, 12 proteins, 13 and polysaccharides 14 can exquisitely control the inorganic crystal type and hierarchical material self-assembly.…”