Lithium-sulfur batteries based on nitrogen-doped carbon and an ionic-liquid electrolyte

Nitrogen-doped mesoporous carbon (NC) and sulfur were used to prepare an NC/S composite cathode, which was evaluated in an ionic-liquid electrolyte of 0.5 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in methylpropylpyrrolidinium bis(trifluoromethane sulfonyl)imide ([MPPY][TFSI]) by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and cycle testing. To facilitate the comparison, a C/S composite based on activated carbon (AC) without nitrogen doping was also fabricated under the same conditions. Compared with the AC/S composite, the NC/S composite showed enhanced activity toward sulfur reduction, as evidenced by the lower onset sulfur reduction potential, higher redox current density in the CV test, and faster charge-transfer kinetics, as indicated by EIS measurements. At room temperature under a current density of 84 mA g^-1 (C/20), the battery based on the NC/S composite exhibited a higher discharge potential and an initial capacity of 1420 mAh g^-1, whereas the battery based on the AC/S composite showed a lower discharge potential and an initial capacity of 1120 mAh g ^-1. Both batteries showed similar capacity fading with cycling due to the intrinsic polysulfide solubility and the polysulfide shuttle mechanism; capacity fading can be improved by further cathode modification. NC vs. AC: Nitrogen-doped mesoporous carbon (NC) is prepared by heat treatment of mesoporous carbon under NH₃ at 850 °C. Compared with KOH-activated mesoporous carbon (AC), NC shows catalytic activity towards sulfur reduction. Furthermore, under the same current rates, NC/S composite shows a higher discharge potential and capacity than the AC/S composite in an ionic-liquid electrolyte both at 25 °C and 50 °C.