Ionic Liquid-Derived Nitrogen-Enriched Carbon/Sulfur Composite Cathodes with Hierarchical Microstructure—A Step Toward Durable High-Energy and High-Performance Lithium–Sulfur Batteries

We describe the preparation of turbostratic carbon in monolithic form by silica template method and its use as host matrix to trap polysulfides in Li–S batteries. The synthesized ionic liquid-derived carbon is hierarchically structured with pore size maxima around 6 nm and 750 nm, and it has a room temperature electrical conductivity of 7–8 S cm –1, owing to a high content of pyridinic and graphitic nitrogen. Further, this carbon can accommodate up to ∼80% sulfur by weight, and the resulting nanocomposite demonstrates promising performance as novel electrode material for Li–S batteries. Cathodes with low areal mass loading (1 mg sulfur cm –2 ) can be cycled at C/5 and 1C over several hundreds of cycles with low polarization and very little capacity fading (4% between the fifth and 1000th cycles at 1C). Highly loaded cathodes (4 mg sulfur cm –2 ) also exhibit good cyclability at C/5 with areal capacities of 2.6 mAh cm –2 on average over 200 cycles. Yet the high capacities make the cells more prone to electrolyte decomposition, which eventually gives rise to low round trip energy efficiencies depending on the electrolyte-to-sulfur mass ratio. Results from in operando X-ray diffraction show that nanocrystalline Li 2 S forms midway through the second discharge plateau, and sulfur recrystallizes in the metastable β-S 8 phase, with 30 to 40 nm crystallites, but undergoes amorphization under open circuit conditions. Overall, this work represents a step forward in the development of durable high-energy Li–S batteries and provides valuable insights into the operation of such cells.