First-principles investigation of the effect of Co in stabilizing the structures of layer-structured cathodes in delithiated state

The structural and chemical features of delithiated layer-structured cathodes (Li0.333TMO2, TM = Ni and Co) are investigated. Energetically stable structures are evaluated by combing particle swarm optimization algorithm and density functional theory calculations. The concentration of defects is calculated assuming that the entire crystal is the statistical combination of the lowest-energy structures. TM antisites that occupy Li sites and interstitial Li ions at tetrahedral sites are the major defect types in Li0.333TMO2; their prevalence decreases with increase in Co content. The oxidation state of Ni ions increases in the presence of Co, because the Co ions sustain low oxidation numbers, which results in decrease in the possibility of Ni2+ formation. In addition, the covalent bond of Co-O appears to be stronger than that of Ni-O. Therefore, the suppression of Ni2+ formation and the strong covalent bonding are proposed as the mechanisms underlying the stabilizing effect of Co.