N/S-Co-doped Porous Carbon Nanoparticles Serving the Dual Function of Sulfur Host and Separator Coating in Lithium–Sulfur Batteries

Porous carbon nanoparticles (PCNs) co-doped with nitrogen and sulfur have been produced by applying a straightforward template-free method entailing a high-temperature reaction between polypyrrole nanoparticles and sodium thiosulfate. The activation process gives rise to porous carbon nanoparticles that combine several important properties: (a) a uniform size of ∼80 nm; (b) a well-developed porosity with BET surface areas of up to ∼1700 m2 g–1 and pore volumes of up to 2.20 cm3 g–1; (c) a sizable N and S heteroatom content (of up to 2.7% N and 7.7% S); and (d) a good electrical conductivity (up to 3 S cm–1). The synthesis strategy offers a great versatility since two types of materials, PCN or S/PCN (nanocomposites comprising elemental sulfur infiltrated into the PCN), can be produced by introducing minor changes to the procedure. These materials have been tested on two components of a lithium–sulfur cell. The S/PCN nanocomposite is used as the cathode, whereas the PCN material is deposited onto the separator to form a thin packed layer in order to restrict the mobility of the polysulfides. Remarkably, the PCN coating layer notably enhances the utilization of sulfur (increase of 23% during the first cycles), and it provides robustness during long-term cycling. The battery assembled with these two components exhibits a highly stable cycling performance from the very first charge–discharge cycles and delivers a reversible capacity of 841 mAh g–1 after 100 cycles at 0.2C with a Coulombic efficiency of 99.3%. Despite using a S/PCN composite with a high sulfur content (>70%), the cell was successfully cycled at 2C over 500 charge–discharge cycles and experienced a capacity decay of only 0.089% per cycle.