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 similar to 9.0 eV to similar 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.