Understanding the interaction of N-doped graphene and sulfur compounds in a lithium–sulfur battery: a density functional theory investigation†
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 (Li2Sx, 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 Li2Sx 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 Li2Sx. The binding of Li2Sx 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.