Water intercalation of sodium cobalt oxides using aqueous permanganate solution

Abstract

We report direct synthesis of the potassium sodium cobalt oxyhydrates (Na,K) x (H 2O) y CoO 2− δ using the aqueous KMnO 4 solution. Two distinct novel phases of (Na,K) x (H 2O) y CoO 2− δ are synthesized depending on the molar ratio of KMnO 4/Na used to treat γ-Na 0.7CoO 2. Low molar ratio of KMnO 4/Na leads to a superconductive phase with c≈19.6 Å, whereas high molar ratio of KMnO 4/Na leads to a non-superconductive phase with c≈13.9 Å. According to the chemical analyses, it is found that a partial or even almost complete replacement of K + for Na + in the alkaline layers. When using aqueous KMnO 4 solution with low molar ratio of KMnO 4/Na, the water intercalation is very slow. We also present the XANES spectra on (Na,K) x (H 2O) y CoO 2− δ . The oxidation state of the Co for the 0.3X sample ( c≈19.6 Å) is higher than that of the 40X sample ( c≈13.9 Å) with a chemical shift of 3.5 eV.