Qiulei Su

Modelling and simulation of electron-rich effect on Li diffusion in group IVA elements (Si, Ge and Sn) for Li ion batteries

Improvements in the electrical conductivity and lithium (Li) mobility for Li ion batteries are of particular importance for their high-power applications. Mapping of electron energy loss spectroscopy shows that the electrochemical reaction front region is under electron-rich conditions during lithiation. In this paper, the electron-rich effect on the diffusion behaviors of Li in pristine and phosphorus-doped group IVA elements, e.g., silicon, germanium and tin, were investigated using the first principles density functional theory (DFT) calculations in combination with a climbing-image nudged elastic band and ab initio DFT molecular dynamics. Phosphorus doping was found to be a non-critical factor for enhanced Li diffusion into Si. Instead, the results showed that the diffusion barriers and diffusivity of Li are mainly affected by the electron-rich effect, i.e. the energy barriers decrease and diffusivity increases in an electron-rich environment. The decrease in diffusion barriers was attributed to the relaxation of Si–Si bonds with extra electrons, which can also apply to the case of Ge but not for metallic Sn. These new findings provide a theoretical and experimental basis for the design and fabrication of next generation batteries with a high power density.

Comparison of Tetragonal and Cubic Tin as Anode for Mg Ion Batteries

Using first-principles calculation based on density functional theory, diffusion of Mg atom into α- and β-Sn was investigated. The diffusion barriers are 0.395 and 0.435 eV for an isolated Mg atom in the α- and β-Sn, respectively. However, the diffusion barriers of the Mg atom decrease in the α-Sn, whereas they increase in the β-Sn, when an additional Mg atom was inserted near the original diffusing Mg atom, which is mainly due to strong binding of Mg-Mg atoms in the β-Sn. Therefore, it is better to use the α-Sn, rather than the β-Sn, as an anode material for Mg ion batteries.

First-principles study of nitrogen adsorption and dissociation on α-uranium (001) surface

The adsorption and dissociation of nitrogen on the α-uranium (001) surface have been studied with a first-principles density functional theory (DFT) approach. The effects of strong 5f electron–electron correlation and spin–orbit coupling on the adsorption of nitrogen on the uranium (001) surface are also discussed. Different coverages of nitrogen atoms and different initial configurations of nitrogen molecules are considered on the uranium surface. The structural parameters and electronic states of nitrogen on the uranium surface are obtained. The calculated results indicate that nitrogen atoms are energetically favorable at the hollow1 sites. The nitrogen molecules adsorbed horizontally on the long-bridge site are found to dissociate completely, and the corresponding adsorption energies are about −4 eV. The electron structure of the most preferred adsorption configuration is investigated, and it is found that the adsorbed nitrogen atoms only seize electrons from the top-most uranium layer. Based on ab initio atomistic thermodynamics, the surface phase diagram for nitrogen adsorption on the α-uranium (001) surface is obtained and the initial stages of nitridation for the uranium surface are discussed.

First-principles study on the interaction of nitrogen atom with α–uranium: From surface adsorption to bulk diffusion

Experimental studies of nitriding on uranium surfaces show that the modified layers provide considerable protection against air corrosion. The bimodal distribution of nitrogen is affected by both its implantation and diffusion, and the diffusion of nitrogen during implantation is also governed by vacancy trapping. In the present paper, nitrogen adsorption, absorption, diffusion, and vacancy trapping on the surface of and in the bulk of α–uranium are studied with a first-principles density functional theory approach and the climbing image nudged elastic band method. The calculated results indicate that, regardless of the nitrogen coverage, a nitrogen atom prefers to reside at the hollow1 site and octahedral (Oct) site on and below the surface, respectively. The lowest energy barriers for on-surface and penetration diffusion occur at a coverage of 1/2 monolayer. A nitrogen atom prefers to occupy the Oct site in bulk α–uranium. High energy barriers are observed during the diffusion between neighboring Oct sites. A vacancy can capture its nearby interstitial nitrogen atom with a low energy barrier, providing a significant attractive nitrogen-vacancy interaction at the trapping center site. This study provides a reference for understanding the nitriding process on uranium surfaces.