Enhancing Thermoelectric Figure-of-Merit of Polycrystalline Na _y CoO₂ by a Combination of Non-stoichiometry and Co-substitution

Abstract

Co-oxides with a layered structure are of interest for high-temperature thermoelectric applications as they can be tuned to enhance their electrical conductivity while retaining their low thermal conductivity. The figure-of-merit of Na _y CoO₂ has been enhanced using the combined effects of Na-non-stoichiometry and non-isoelectronic Co-substitution. A series of compounds Na_0.7Co_1−xNi _x O₂ with x ≤ 0.1 have been synthesized using conventional techniques. Structural analysis using x-ray diffraction and Rietveld refinement shows the formation of a γ-NaCoO₂-type phase in all the compounds. The presence of a small amount of NiO for x > 0.05 indicates that the solubility limit of Ni in Na_0.7CoO₂ is 5 at.%. All the compounds have been found to be p-type with the thermopower reaching a maximum of 220 μV K⁻¹ at 1023 K for x = 0.1. The thermopower has been found to vary linearly with temperature for all the compounds; a degenerate metallic behavior. The electrical resistivity varies between 3 and 10 mΩ cm at all temperatures and has a metallic temperature dependence in agreement with the thermopower results. The power factor for the x = 0.1 compound reaches a maximum value of 0.55 mW m⁻¹ K⁻² at ∼ 900 K compared to 0.45 mW m⁻¹ K⁻² for the compound with no substitution. The thermal conductivity at 1023 K decreases from 1.2 to 0.9 W m⁻¹ K⁻¹ for x = 0.1. These factors lead to an increase of the figure-of-merit, zT, to 0.58 at 1023 K for x = 0.1, an increase of 57% compared to the unsubstituted compound. The magnetic studies show that Na_0.7CoO₂ is paramagnetic with an antiferromagnetic transition at ∼ 36 K. Substitution of Ni²⁺ for Co³⁺ has been found to induce a ferromagnetic-like transition at ∼ 240 K which is suppressed at high fields.