Hubbard-corrected Density Functional Theory Research Articles

A Density Functional Theory Study of the Ionic and Electronic Transport Mechanisms in LiFeBO3 Battery Electrodes

Lithium iron borate is an attractive cathode material for Li-ion batteries due to its high specific capacity and low-cost, earth-abundant constituents. However, experiments have observed poor electrochemical performance due to the formation of an intermediate phase, that is, LixFeBO3, which leads to large overvoltages at the beginning of charge. Using a convex-hull analysis, based on Hubbard-corrected density functional theory (DFT+U), we identify this intermediate phase as Li0.5FeBO3. Moreover, we show by means of the nudged elastic band (NEB) method, that the origin of these adverse electrochemical effects can be explained by an intrinsically low Li-ion and electron/hole-polaron mobility in Li0.5FeBO3 due to high activation barriers for both the ionic and electronic transport. These studies include the effects of the experimentally reported commensurate modulation. We have also investigated the Li-ion/hole diffusion through the interface between Li0.5FeBO3 and LiFeBO3, which is found not to result in ad...

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.

Role of Cluster Morphology in the Dynamics and Reactivity of Subnanometer Pt Clusters Supported on Ceria Surfaces

We study subnanometer (sub-nm) Pt clusters supported by highly reducible oxide surfaces and establish the role of cluster morphology in the thermodynamics and kinetics of surface processes relevant for reactivity, namely cluster mobility, reverse oxygen spillover, and oxygen vacancy formation. The relationships between cluster morphology and reactivity are rarely considered in computational studies because of the large domain and complexity of the potential energy surface, particularly in the presence of strong metal–support interaction. Global optimization algorithms together with Hubbard-corrected density functional theory calculations (DFT+U) are used to identify the stable and metastable morphologies of Pt3–Pt6 clusters supported on pristine and defective CeO2(111) surfaces. Our systematic exploration for these sub-nm Pt particles shows that the charge of the supported cluster, its bonding to the substrate, and the degree of ceria reduction depend on the metal/oxide interface area and on the cluster morphology. Concerning reaction thermodynamics and kinetics, the use of global optimization methods leads to very different results as compared to usual minimization procedures. By allowing for morphology changes during reaction, the energetics of reverse O spillover changes from highly endothermic to exothermic and leads to new minimum-energy reaction and diffusion mechanisms. The diffusion kinetics predicts clusters as small as Pt6 to be resistant to sintering on ceria surfaces. The relevance of these findings for larger metal clusters and for supporting oxide nanoparticles is discussed as well as their connection with the recent literature.