Electrochemical de-intercalation, oxygen non-stoichiometry, and crystal growth of NaxCoO2−δ

We report a detailed study of de-intercalation of Na from the compound NaxCoO2−δ using an electrochemical technique. We find evidence for stable phases with Na contents near the fractions x≃1/3, 1/2, 5/8, 2/3, and 3/4. Details regarding the floating-zone crystal growth of Na0.75CoO2 single crystals are discussed as well as results from magnetic susceptibility measurements. We observe the presence of significant oxygen deficiencies in powder samples of Na0.75CoO2−δ prepared in air, but not in single crystal samples prepared in an oxygen atmosphere. The oxygen deficiencies in a Na0.75CoO2−δ sample with δ∼0.08 remain even after electrochemically de-intercalating to Na0.3CoO2−δ.