Synthesis of monolayers and bilayers of cobalt oxyhydrates (Na,K) x (H2O) y CoO2-

Abstract

Potassium sodium cobalt oxyhydrates (Na,K)x(H2O)yCoO2- were synthesized from -Na0.7CoO2 by using aqueous KMnO4 solution in a one-pot process. Chemical and structural analyses revealed that a partial or even almost complete replacement of K+ for Na+ in the alkaline layers occurs. Direct formation of the c approximate to 13.9 angstrom phase is apparently associated with the larger size of K+ (1.4 angstrom) as compared to Na+ (1.0 angstrom). Formation of (Na,K)x(H2O)yCoO2- not only involves de-intercalation, oxidation and hydration processes, but also an ion exchange reaction. Based on a systematic study, the phase formation of (Na,K)x(H2O)yCoO2- with c approximate to 19.6 angstrom is a slow process, particularly when using aqueous KMnO4 solution with low molar ratio of KMnO4/Na. When comparing the Co K-edge X-ray absorption spectra of (Na,K)x(H2O)yCoO2- with those of Nax(H2O)yCoO2 obtained from Br2/CH3CN solution, the edge energy of the main peak of the bilayered hydrate is found to be 3.5 eV higher than that of the monolayered hydrate for (Na,K)x(H2O)yCoO2-. In contrast, the edge energy of the main peak of the bilayered hydrate is 0.4 eV lower than that of the monolayered hydrate for Nax(H2O)yCoO2. In addition, the hydration behavior of monolayered of (Na,K)x(H2O)yCoO2- is different from that of Nax(H2O)yCoO2. These results seem to suggest that they are two different systems.