In situ sulfur-doped graphene nanofiber network as efficient metal-free electrocatalyst for polysulfides redox reactions in lithium–sulfur batteries

The major challenge for realistic application of Li-S batteries lies in the great difficulty in breaking through the obstacles of the sluggish kinetics and polysulfides shuttle of the sulfur cathode at high sulfur loading for continuously high sulfur utilization during prolonged charge-discharge cycles. Here we demonstrate that large percentage of sulfur can be effectively incorporated within a three-dimensional (3D) nanofiber network of high quality graphene from chemical vapor deposition (CVD), through a simple ball-milling process. While high quality graphene network provided continuous and durable channels to enable efficient transport of lithium ions and electrons, the in-situ sulfur doping from the alloying effect of ball milling facilitated desirable affinity with entire sulfur species to prevent sulfur loss and highly active sites to propel sulfur redox reactions over cycling. This resulted in remarkable rate-performance and excellent cycling stability, together with large areal capacity at very high sulfur mass loading (Specific capacity over 666 mAh g⁻¹ after 300 cycles at 0.5 C, and areal capacity above 5.2 mAh cm⁻² at 0.2 C at sulfur loading of 8.0 mg cm⁻² and electrolyte/sulfur (E/S) ratio of 8 µL mg⁻¹; and high reversible areal capacities of 13.1 mAh cm⁻² at a sulfur load of 15 mg cm⁻² and E/S of 5 µL mg⁻¹).