First-principles study on the effect of torsional deformation on WSe as an anode material for calcium ion batteries.

CONTEXT

In this paper, the effects of torsional deformation on the electronic properties of intrinsic WSe system and Ca-adsorbed WSe system were systematically studied by first-principles method. The results show that Ca can be stably adsorbed on the vacancy (H site) of WSe surface in all deformation systems, and the adsorption energy of the system without deformation is the highest. Intrinsic WSe is a semiconductor with a direct band gap of 1.53 eV. The torsional deformation makes WSe change from a direct band gap semiconductor to an indirect band gap semiconductor and finally to a metal property. The adsorption of Ca makes the conduction band of WSe move down and increases the number of peaks in the conduction band region. The new density of state peaks are mainly derived from the contribution of W-d, Se-p, and d orbitals of adsorbed atoms in each adsorption system. Mulliken charge analysis shows that Ca transfers most of the valence electrons to the substrate, and the torsional deformation changes the amount of transferred charge. The twist deformation reduces the diffusion barrier of Ca on WSe surface from 0.20 to 0.14 eV. The above results provide a basis for the improved application of WSe in ion batteries.

METHODS

In this study, all the first-principles calculations are based on Materials Studio 8.0 software package. The generalized gradient approximation (GGA) functional Perdew-Burke-Ernzerhof (PBE) is used for the electron exchange correlation interactions in all systems. The optimization algorithm uses Broyden-Fletcher-Goldfarb-Shanno (BFGS) to optimize the model structure and calculate the energy. The measured cutoff energy is optimized to 450 eV, and the radius of the vacuum layer in the Z-axis direction is 20 Å. The K-point of 7 × 7 × 1 is selected by Monkhorst-Pack method. The structural optimization criterion is selected, the convergence radius of the force is 0.01 eV/Å, and the displacement radius between atoms is within 0.001 Å distance. The energy convergence radius of each atom is less than 1.0 × 10 eV/atom.