An ab initio study of the interaction of a single Li atom with single-walled SiGe (6,6) nanotubes and consequences of Jahn–Teller effect

Abstract

A systematic study of a Li atom and four types of SiGe (6,6) nanotubes (NTs) of length ~20 A has been performed to explore SiGeNTs’ viability as anode material in Li-ion batteries. For all structures, full geometry and spin optimizations were performed without any symmetry constraints. We used the hybrid functional B3LYP and an all electron 6-311G**//3-21G* basis set implemented in GAUSSIAN 09 suite of software. All possible internal and external adsorption sites were considered, and it was found that some tubes deformed as a result of the Jahn–Teller effect. Out of all four types, type III NTs were found to retain the tubular shape for most cases of adsorption. Our calculations predict that SiGeNTs attract Li atom on top of Si, on top of Ge, and on top of hollow sites with adsorption energies ranging from 0.888 to 1.657 eV. Most probable sites for external and internal adsorptions were quasi on top of Ge site and quasi hollow site, respectively. After adsorption of the Li atom, Jahn–Teller active orbitals of the systems vanished from the frontier orbital region, indicating that more Li atoms can be adsorbed while keeping the desired tubular shape. In general, the difference between the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level decreased with respect to the bare SiGeNT values. The decreased HOMO–LUMO gaps vary between 0.685 and 0.930 eV. Charge population analysis indicated that charge generally accumulated around the nearest neighbor atoms to the Li atom. We conclude that with the careful placement of Li atoms, SiGeNTs can be very successful as anode material in Li-ion battery technology.