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 P63/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.