Understanding the interaction of N-doped graphene and sulfur compounds in a lithium–sulfur battery: a density functional theory investigation

Abstract

The generalized gradient approximation (GGA) density functional theory (DFT), Perdew–Burke–Ernzerh (PBE), and long-range corrected DFT (ωB97XD) were used to investigate the interaction between lithium polysulfides (Li₂S_x, x = 2–8) and N-doped graphene to understand the adsorption mechanism and effect of catalytic conversion, including the reduction from S₈ to Li₂S and Li₂S decomposition. The binding of Li₂S_x to N-doped graphene was significantly stronger than that to solvents, such as 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME), approximately by 0.5–3.0 eV, indicating that N-doped graphenes possessed the ability to trap soluble Li₂S_x. The binding of Li₂S_x to N-doped graphenes followed the order of pyrrolic N > pyridinic N > graphene > graphitic N. Among them, pyrrolic N exhibited the strongest anchoring effect mainly via forming the coordination bond of LiN₃. In addition, in all the investigated systems, the van der Waals force also played an important role in binding. We comprehensively analyzed the micro-nano-scale theoretical data, including charge, bond distance, bond order, electron density difference, the electron localization function (ELF), and the independent gradient model (IGM) of the Li ion and N atom. The strength of the Li–N bond confirmed that the strongest anchoring occurred when the Li ion was trapped above the vacancy-forming coordination bond rather than a regular covalent bond. Although the strength of Li–N binding had a direct effect on the binding energy, other thermodynamic parameters (free energy and dissociation energy) of catalytic conversion and the energy barrier of Li₂S phase transformation also showed a similar capacity.