Successes and failures of Hubbard-corrected density functional theory: The case of Mg doped LiCoO$_2$

TL;DR

This study assesses the effectiveness of Hubbard-corrected DFT in modeling Mg doping in LiCoO$_2$, revealing how the U parameter influences electronic and structural predictions, especially regarding impurity states.

Contribution

It systematically analyzes how the U parameter affects the properties of Mg-doped LiCoO$_2$, highlighting challenges in accurately modeling doped transition metal oxides.

Findings

Impurity states cause Co valency changes and U-dependent property shifts.

Impurity-state-free complexes are U-insensitive.

Difficulty in matching experimental properties due to U dependence.

Abstract

We have evaluated the successes and failures of the Hubbard-corrected density functional theory (DFT+U) approach to study Mg doping of LiCoO$_2$. 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 LiCoO$_2$. We find that formation of impurity states results in changes on the valency of Co in LiCoO$_2$. 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 LiCoO$_2$.