Experimental and First-Principles Thermodynamic Study of the Formation and Effects of Vacancies in Layered Lithium Nickel Cobalt Oxides

The formation of vacancies and the structural stability of layered lithium nickel oxide (LNO)-based cathode materials are investigated. The thermodynamic stability of oxygen and lithium vacancies and their most stable configurations are examined by first-principles density functional theory calculations. The underlying chemical mechanism is analyzed by a molecular orbital method. The weaker ionic bonding between Ni and O than between Co and O is found to be the main cause for the imperfect structure of LNO crystals. On the basis of these calculations, phase diagrams of the Li-(Ni,Co)-O system were simulated. The crystals containing vacancies are included as independent phases in the simulation. This approach enabled investigation of the relationship between the processing conditions and vacancy formation. The O and Li vacancy pairs are simulated to appear with high temperature processing. On the basis of the calculation of energy barriers, we speculate that these vacancy pairs provide an alternative migration route for Ni ions, which causes the observed structural instability. The effect of oxygen partial pressure was also examined. The first-principles calculation results were compared with experimental results, which showed excellent agreement confirming the validity of the models and calculation methods used in this study.