The Effective Design of a Polysulfide-Trapped Separator at the Molecular Level for High Energy Density Li-S Batteries.

In this work, the lightweight and scalable organic macromolecule graphitic carbon nitride (g-C3N4) with enriched polysulfide adsorption sites of pyridinic-N was introduced to achieve the effective functionalization of separator at the molecular level. This simple method overcomes the difficulty of low doping content as well as the existence of an uncontrolled form of nitrogen heteroatom in the final product. Besides the conventional pyridinic-N-Li bond formed in the vacancies of g-C3N4, the C-S bond was interestingly observed between g-C3N4 and Li2S, which endowed g-C3N4 with an inherent adsorption capacity for polysulfides. In addition, the microsized g-C3N4 provided the coating layer with good mechanical strength to guarantee its restriction function for polysulfides during long cycling. As a result, an excellent reversible capacity of 840 mA h g(-1) was retained at 0.5 C after 400 cycles for a pure sulfur electrode, much better than that of the cell with an innocent carbon-coated separator. Even at a current density of 1 C, the cell still delivered a stable capacity of 732.7 mA h g(-1) after 500 cycles. Moreover, when further increasing the sulfur loading to 5 mg cm(-2), an excellent specific capacity of 1134.7 mA h g(-1) was acquired with the stable cycle stability, ensuring a high areal capacity of 5.11 mA h cm(-2). Besides the intrinsic adsorption ability for polysulfides, g-C3N4 is nontoxic and mass produced. Therefore, a scalable separator decorated with g-C3N4 and a commercial sulfur cathode promises high energy density for the practical application of Li-S batteries.