First principles prediction of nitrogen-doped carbon nanotubes as a high-performance cathode for Li–S batteries

The insulating nature of sulfur and the solubility of polysulfide in an organic electrolyte are two main factors that limit the application of lithium sulfur (Li–S) battery systems. Enhancement of Li conductivity, identification of a strong adsorption agent for polysulfides and the improvement of the whole sulfur-based electrode are of great technological importance. The diffusion of Li atoms in the outer-wall, inner-wall and inter-wall spaces in nitrogen-doped double-walled carbon nanotubes (CNTs) and penetrations of Li and S atoms through the walls are studied using density functional theory. We find that N-doping does not alter the diffusion behavior of Li atoms throughout the CNTs, but the energy barrier for Li atoms to penetrate the wall is greatly decreased by N-doping (from ∼9.0 eV to ∼1.0 eV). On the other hand, the energy barrier for S atoms to penetrate the wall remains very high, which is caused by the formation of chemical bonds between S and nearby N atoms. The results indicate that Li atoms are able to diffuse freely, whereas S atoms can be encapsulated inside the N-doped CNTs, suggesting that the N-doped CNTs can be potentially used in high performance Li–S batteries.