Understanding the modulation effect and surface chemistry in a heteroatom incorporated graphene-like matrix toward high-rate lithium-sulfur batteries

The underlying interface effects of sulfur hosts/polysulfides at the molecular level are of great significance to achieve advanced lithium-sulfur batteries. Herein, we systematically study the polysulfide-binding ability and the decomposition energy barrier of Li2S enabled by different kinds of nitrogen (pyridinic N, pyrrolic N and graphitic N) and phosphorus (P-O, PO and graphitic P) doping and decipher their inherent modulation effect. The doping process helps in forming a graphene-like structure and increases the micropores/mesopores, which can expose more active sites to come into contact with polysulfides. First-principles calculations reveal that the PO possesses the highest binding energies with polysulfides due to the weakening of the chemical bonds. Besides, PO as a promoter is beneficial for the free diffusion of lithium ions, and the pyridinic N and pyrrolic N can greatly reduce the kinetic barrier and catalyze the polysulfide conversion. The synergetic effects of nitrogen and phosphorus as bifunctional active centers help in achieving an in situ adsorption-diffusion-conversion process of polysulfides. Benefiting from these features, the graphene-like network achieves superior rate capability (a high reversible capacity of 954 mA h g-1 at 2C) and long-term stability (an ultralow degradation rate of 0.009% around 800 cycles at 5C). Even at a high sulfur loading of 5.6 mg cm-2, the cell can deliver an areal capacity of 4.6 mA h cm-2 at 0.2C.