Sulfur Redox Catalysis of Cobalt-Embedded Nitrogen-Doped Carbon Nanotube-Functionalized Separators in Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) offer high specific energy at low cost but are hindered by the shuttle effect, which reduces capacity and cycle life due to sluggish sulfur (S) redox kinetics. Although the issue can be addressed by modifying the separator with metal–nitrogen–carbon (M–N–C) materials, detailed explanations are often lacking. In this study, cobalt-embedded nitrogen-doped carbon nanotubes (Co-NCNTs) were used to functionalize the separator. X-ray photoelectron spectroscopy shows that Li+ from lithium polysulfides (LiPSs) interacts with the N–C framework in Co-NCNTs during discharge, accompanied by electron injection to maintain the charge balance. This electron exchange is more pronounced at the Co and N sites throughout the charge/discharge cycle. This can facilitate the rate-determining conversion between Li2S and polysulfides upon interaction with Co-NCNTs. As a result, our modified LSBs delivered 1496.9 and 501.1 mA h g–1 at discharge rates of 0.1 and 5 C, respectively, maintaining stability over 100 cycles without significant shuttle effect. This work provides valuable insights into designing high-energy, long-life LSBs through separator modification.