An atomic/molecular-level strategy for the design of a preferred nitrogen-doped carbon nanotube cathode for Li-O2 batteries

Abstract

The formation and growth of Li2O2, the primary discharge product with extremely low conductivity, will cause the passivation of the cathode and the failure or even death of Li-O2 batteries. In this paper, by means of molecular dynamics and density functional theory, we systematically investigated the morphology evolution and electron transport pathway of (Li2O2)n on the surface of nitrogen-doped carbon nanotubes (NCNTs) and the effects of the possible N configuration on the growth stability of (Li2O2)n. Our results demonstrated that the morphology of (Li2O2)n on the surface of NCNTs undergos a process from scattered small-sized clusters to film-like (axial) and toroidal (circumferential) large clusters, and finally completely surrounded the NCNTs. Compared with graphitic N1 and graphitic N2, Li2O2 tends to aggregate stably on the surface of pyridinic N3 and pyridinic N4, which can effectively reduce the capacity loss caused by Li2O2 branches and free state Li2O2. In addition, a stable gap exists between Li2O2 and NCNTs, which is attributed to the Coulomb force and van der Waals interaction. Importantly, (Li2O2)n with p-type semiconductor characteristics acts as a bridge between NCNTs, and electrons are mainly transferred along the surface of (Li2O2)n clusters. Moreover, increasing the wall number of NCNTs and using NCNT bundles were found to be beneficial to improving the deposition stability of (Li2O2)n. Notably, pyrrolic N3 with unstable structure is not recommended for practical application. These results offer general guidelines for designing highly active catalysts for Li-O2 batteries in terms of the optimization of the NCNT-based cathode, and the unique research methods also shed more light on the analysis of the discharge product behavior of other potential metal-air batteries.