Na-site substitution effects on the thermoelectric properties ofNaCo2O4

Abstract

The resistivity and thermopower of Na1+xCo2O4 and Na1.1−xCaxCo2O4 are measured and analyzed. In Na1+xCo2O4, whereas the resistivity increases with x, the thermopower is nearly independent of x. This suggests that the excess Na is unlikely to supply carriers, and decreases effective conduction paths in the sample. In Na1.1−xCaxCo2O4, the resistivity and the thermopower increase with x, and the Ca 2+ substitution for Na + reduces the majority carriers in NaCo2O4. This means that they are holes, which is consistent with the positive sign of the thermopower. Strong correlation in this compound is evidenced by the peculiar temperature dependence of the resistivity.PACS numbers: 72.15. Jf, 72.80.Ga, 72.15.LhThere appears a growing interest to a hunt for new thermoelectric (TE) materials, 1 reflecting urgent needs for a new energy-conversion system in harmony with our environments. A TE material generates electric power in the presence of temperature gradient through the Seebeck effect, and pumps heat in the presence of electric current through the Peltier effect. A serious drawback is the low conversion efficiency: It is characterized by the so-called "figure of merit" Z = S 2 /ρκ, where S, ρ and κ are the thermopower, resistivity and thermal conductivity of a TE material, respectively. In other words, a good TE material is a material that shows large S, low ρ and low κ. However, a high value of Z is difficult to realize, because the three parameters cannot be changed independently. To overcome this difficulty, a number of new concepts and new materials have been examined.Recently we have observed that a layered cobalt oxide NaCo 2 O 4 exhibits unusually large S (100 µV/K at 300 K) accompanied by low ρ (200 µΩcm at 300 K) along the direction parallel to the CoO 2 plane. 2 NaCo 2 O 4 belongs to a layered Na bronze Na x CoO 2 , which was studied as a cathode for sodium batteries. 3 During the characterization, Molenda et al. 4 first found a large S in Na 0.7 CoO 2 . Although they noticed that S was anomalously large, they did not mention a possibility for a TE material. Their samples were polycrystals, the resistivity of which was 2-4 mΩcm at 300 K, much higher than that of our crystals. Our finding is that the carrier density (n) is of the order of 10 21 − 10 22 cm −3 , and is two orders of magnitude larger than n of conventional TE materials. This is difficult to understand in the framework of a conventional one-electron picture, and may indicate a way to get a good TE material other than the conventional approach. We have proposed that strong electron-electron correlation plays an important role in the enhancement of the thermopower of NaCo 2 O 4 .Even in a correlated system, we can expect that a conductor of low n will have a large S, because the diffusive part of S is the transport entropy, of the order of k B T /E F , where E F is the Fermi energy. 5 Thus it would be tempting to improve the TE properties in NaCo 2 O 4 by decreasing n. We easily think of three ways to change n in NaCo 2 O 4 , i.e., (i) doping of e...

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