Sodium intercalation in NaxCoO2-y - Correlation between crystal structure, oxygen nonstoichiometry and electrochemical properties

In this work two different NaxCoO2 (- y) oxides (Na0.69CoO2 (- y) and Na0.72CoO2 (-) (y)) were studied in terms of their crystal structure, sodium content, oxygen nonstoichiometry, transport properties (electrical conductivity and thermoelectric power) and electrochemical properties in Na/Na+/NaxCoO2 - y cells. The materials were prepared by a solid-state method. XRD measurements with Rietveld analysis confirmed the presence of a single phase with P6(3)/mmc symmetry. Oxygen nonstoichiometry, which was calculated on a basis of thermogravimetric reduction experiments was found to be small, not exceeding 0.04 mol.mol(-1). Electrical conductivity for both compounds shows metallic-type character, while values of thermoelectric power are high at room temperature, but decrease considerably for lower temperatures. Electrochemical studies performed for Na/Na+/NaxCoO2 (- y) cells in 0.68 <= x <= 0.81 range indicated the presence of a step-like voltage curve with two characteristic voltage plateaus visible at potentials of 2.54 V and 2.44 V vs. metallic Na electrode. Crystal structure and deviation from the oxygen stoichiometry in the studied materials were related to the character of the discharge curve of Na/Na+/NaxCoO2 (- y) cells and changes of transport properties of NaxCoO2 (- y) oxides as a function of sodium content x and oxygen nonstoichiometry y. (C) 2013 Elsevier B.V. All rights reserved.