Theoretical analysis of oxygen vacancies in layered sodium cobaltate   Na_xCoO_{2-\delta}
  Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in Na_xCoO_2 with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications. 
Simone Casolo, Ole Martin L{\o}vvik, Harald Fjeld and Truls Norby
cond-mat.mtrl-sci