Synthesis of monolayers and bilayers of cobalt oxyhydrates (Na,K)_x(H₂O)_yCoO_2−δ

Abstract

Potassium sodium cobalt oxyhydrates (Na,K)_x(H₂O)_yCoO_{2− δ} were synthesized from γ-Na_0.7CoO₂ 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 ≈ 13.9 Å phase is apparently associated with the larger size of K⁺ (∼1.4 Å) as compared to Na⁺ (∼1.0 Å). Formation of (Na,K) _x (H₂O) _y CoO_{2− δ} 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) _y CoO_{2− δ} with c ≈ 19.6 Å 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) _y CoO_{2− δ} with those of Na _x (H₂O) _y CoO₂ 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) _y CoO_{2− δ}. 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 Na _x (H₂O) _y CoO₂. In addition, the hydration behavior of monolayered of (Na,K) _x (H₂O) _y CoO_{2− δ} is different from that of Na _x (H₂O) _y CoO₂. These results seem to suggest that they are two different systems.

Keywords:

• layered oxides
• phase transformation
• thermogravimetric analysis
• X-ray spectroscopy
• XRD

Acknowledgement

This work is supported by the National Science Council of ROC, grant Nos. NSC 92-2112-M-018-005 and NSC 93-2112-M-018-003.