Comparison of the chemical and structural instabilities of Na0.75-xCoO2 and Li1-xCoO2

With a careful handling of the chemically desodiated Na0.75-xCoO2 [0.32 <=(0.75-x)<= 0.75] samples, the oxidation state of Co is found to be in agreement with the theoretically expected values. The apparent lowering of the oxidation state reported in the literature at low sodium contents is due to a rapid absorption of water from ambient air and the lack of appropriate corrections in the molecular weight used in the oxidation state calculation. However, a significant decrease in the oxidation state of cobalt is found for the bilayer hydrate sample obtained by immersing the anhydrous Na0.75-xCoO2 in water due to a reduction of the highly oxidized Co4+ ions by water and a possible insertion of oxonium (H3O+) ions into the lattice as evident from the redox titration and thermogravimetric analysis data. On the other hand, the analogous layered Li0.3CoO2 does not absorb any water on exposure to air. More importantly, the Li1-xCoO2 system incorporates protons into the lattice during chemical delithiation while the Na0.75-xCoO2 system does not down to a sodium content of 0.32. The differences between the two systems are discussed based on the differences in the structures and the position of the Co3+/4+:3d band relative to the top of the O2-:2p band.