LiCoO2 is currently the preferred cathode material in secondary Li-ion batteries. Stoichiometric LiCoO2 has rhombohedral symmetry and belongs to the R3m space group. This layered structure has Li and Co ions ordered along (111) planes to form alternating cation layers. The Co3+ ion, located in octahedral sites, forms a strong bond with neighboring oxygen atoms to produce O-Co-O sheets. Li layers are sandwiched between these sheets of CoO2.
It has traditionally been accepted that in lithium metal oxides (LiMO2) the valence of the transition metal compensates for the charge on the intercalated Li. The theoretical boundaries for Li concentration in an electrode of LixCoO2 are 0≤x≤l. Using a simple ionic charge transfer model, the fully-lithiated material has ions with charges of Li+, Co3+, and O2-. in the CoO2 end member the ions have charges of Co4+ and O2- This simple model incorrectly leads to the conclusion that the oxygen valence is virtually unaffected by Li intercation.