Successes and failures of Hubbard-corrected density functional theory: The case of Mg doped LiCoO₂

We have evaluated the successes and failures of the Hubbard-corrected density functional theory approach to study Mg doping of LiCoO₂. We computed the effect of the U parameter on the energetic, geometric, and electronic properties of two possible doping mechanisms: (1) substitution of Mg onto a Co (or Li) site with an associated impurity state and (2) formation of impurity-state-free complexes of substitutional Mg and point defects in LiCoO₂. We find that formation of impurity states results in changes on the valency of Co in LiCoO₂. Variation of the Co U shifts the energy of the impurity state, resulting in energetic, geometric, and electronic properties that depend significantly on the specific value of U. In contrast, the properties of the impurity-state-free complexes are insensitive to U. These results identify reasons for the strong dependence on the doping properties on the chosen value of U and for the overall difficulty of achieving agreement with the experimentally known energetic and electronic properties of doped transition metal oxides such as LiCoO₂.