The structure and properties of a carbon nanotube (7, 7) with a vacancy defect

Carbon nanotubes (CNTs) still draw great attention of researchers due to their possible use as anodes for lithium-ion batteries. The capacity of the anode for lithium ions is defined by the efficiency of diffusion of these ions to the adsorption centers located on the outer or inner surface of the nanotubes, as well as between the layers of multi-walled nanotubes. The inner surface can be accessed through the open ends of the tubes or defects in their frame. Moreover, the defects create more efficient adsorption sites than the surface of a perfect tube. This work presents the results of the ab-initio modeling of the lithium adsorption on a carbon nanotube (7, 7) with a vacancy-type defect. The simulation was carried out using density functional theory implemented in the SIESTA package. The Ceperley-Alder exchange-correlation functional and the DZP basis set were used. The formation energies of vacancy defects of two types were determined. Additionally, for each vacancy type, the binding energies of lithium atoms adsorbed inside and outside the CNT (7, 7) near the defect were calculated. These binding energies were shown to be up to twice bigger than on a perfect CNT. The enhancing effect of the vacancy on the CNT sorption activity was found to be wide-ranged: it was observed for adsorption sites in the first and second surroundings of the defect and even for sites located on the tube surface opposite to the defect.