Sodium and oxygen nonstoichiometry and thermoelectric properties of NaxCoO2+d

Abstract

The sodium-cobalt oxide NaxCoO2+d is a promising candidate for thermoelectric applications since it may possess simultaneously large thermoelectric power and low resistivity. It is difficult to understand the simultaneous appearance of the two properties within the framework of conventional one-electron models. It has been suggested that the spin state of cobalt ions plays a crucially important role in enhancing the thermoelectric power. In order to study the relationship between the cobalt spin state and thermoelectric properties, NaxCoO2+d samples with precisely controlled sodium and oxygen contents are indispensable. Recently, we established an uncon-ventional sample-synthesis method that enables us to precisely control the Na content in NaxCoO2+d samples. In this article, we discuss the effects of sodium and oxygen nonstoichiometry on the thermoelectric properties of NaxCoO2+d.