| Abstract
| - We report a complete analysis of the formal Co3+/4+ oxidation state in NaxCoO2, in the interval0.31 ≤ x ≤ 0.69. Iodometric titration and thermoelectric power confirm that a direct relationship betweenthe Na content and the amount of Co3+ cannot be established in this system. Creation of a significantamount of oxygen vacancies accompanies Na-ion deintercalation, keeping the formal Co valence at 3.45+for x ≤ 0.45. In light of new thermoelectric power data which reveals important differences between thehydrated (superconducting) and nonhydrated (nonsuperconducting) samples, we propose here that waterplays an important “chemical” role beyond that of a spacer between the CoO2 layers.
- Schematic of the evolution of the band structure with Na removal. Introduction of holes in the O:2p6 band is made possible for low Na contents, which leads to an oxygen evolution from the structure and the impossibility of oxidation by Na deintercalation. The oxidation state of Co is kept constant (closer to 3+ than expected) due to this redox process.
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