On the metastable O2-type LiCoO2

The exchange of sodium for lithium in P2-Na0.70CoO2 leads to an unusual O2-LiCoO2 variety. Rietveld refinement of the neutron diffraction pattern of the O2-LiCoO2 phase confirms the stoichiometry and the structural parameters previously reported and shows that the strong Li+–Co3+ electrostatic repulsion occurring through the common face of the CoO6 and LiO6 octahedra leads to small displacements of the lithium and cobalt ions from the center of their octahedra. Magnetic measurements and 7Li MAS NMR confirm a composition very close to the ideal one for the O2 phase. The O2-LiCoO2 phase is metastable and transforms to well-crystallized O3-LiCoO2 upon heating. Even at intermediate temperatures, the formation of LT-LiCoO2 phase is never observed. Various morphologies are obtained for the O3-LiCoO2 phases, depending on the thermal treatment used. A preliminary electrochemical study shows that almost all the lithium ions can be reversibly deintercalated from the O2-LiCoO2 phase.