Efficient Activation of High-Loading Sulfur by Small CNTs Confined Inside a Large CNT for High-Capacity and High-Rate Lithium–Sulfur Batteries

Sulfur with a high specific capacity of 1673 mAh g–1 is yet to be used as commercial cathode for lithium batteries because of its low utilization rate and poor cycle stability. In this work, a tube-in-tube carbon structure is demonstrated to relieve the critical problems with sulfur cathode: poor electrical conductivity, dissolution of lithium polysulfides, and large volume change during cycling. A number of small carbon nanotubes (∼20 nm in diameter) and a high loading amount of 85.2 wt % sulfur are both filled completely inside a large amorphous carbon nanotube (∼200 nm in diameter). Owing to the presence of these electrically conductive, highly flexible and structurally robust small CNTs and a large CNT overlayer, sulfur material exhibits a high utilization rate and delivers a large discharge capacity of 1633 mAh g–1 (based on the mass of sulfur) at 0.1 C, approaching its theoretical capacity (1673 mAh g–1). The obtained S-CNTs@CNT electrode demonstrates superior high-rate cycling performances. Large discharge capacities of ∼1146, 1121, and 954 mAh g–1 are observed after 150 cycles at large current rates of 1, 2, and 5 C, respectively.