Sandwich-Structured flexible interlayer with Co3O4 nanoboxes strung along carbon nanofibers on both sides for fast Li+ transport and high redox activity in High-Rate Li-S batteries

The conductivity and Li+ transfer of Co3O4 nanoboxes can be significantly improved by running conductive carbon nanofiber “wires” through them. Herein, a membrane is designed with N-doped carbon (NC) nanofiber core sandwiched between NC nanofibers embedded in Co3O4 nanoboxes (Co3O4@NC). Intertwined carbon nanofibers serve as flexible conductive support for transporting Li+ and electrons. Hollow Co3O4 nanoboxes are strung along the surface nanofibers to form a necklace-like structure, which is robust against volumetric expansion of sulfur, enables high sulfur loading, and provides additional active sites for anchoring lithium polysulfides (LiPSs) and their catalytic reactions. During discharging, LiPSs anchored on Co3O4 surface are rapidly converted to Li2S when Li+ and electrons enter the nanoboxes. During charging, solid Li2S reverts to sulfur on Co3O4 surface while Li+ and electrons leave along the nanofibers. Moreover, strong bonding of Li+ with exposed oxygen atoms on the Co3O4 (3 1 1) plane helps separate Li+ from Li2S and decrease the Li2S dissociation energy barrier. The synergy between fast Li+/electron transfer and high catalytic activity effectively suppresses the shuttle effect and significantly improves the high-rate performance and cycling stability. Li-S cells using this Co3O4@NC-NC-Co3O4@NC interlayer deliver a specific capacity of 703.1 mAh g−1 after 450cycles at 1.0C. Even at a high sulfur loading of 4.94 mg cm−2, the cells maintain 718.2 mAh g−1 after 120cycles at 0.2C.