Phase transformation controlled Co1−xS-CoS2 heterostructures embedded in S-doped carbon nanofibers for superior Sodium-Ion storage

Cobalt-based sulfides are considered as a promising anode material for sodium-ion batteries (SIBs) due to its various chemical formulations and high theoretical capacity. However, slow kinetics and structural instability lead to unsatisfactory electrochemical performance. Herein, ultrafine particles with Co1−xS-CoS2 heterostructures precisely controlled by phase transformation has been integrated with the encapsulation with S-doped carbon nanofiber through the one-stone-two-birds strategy. Due to the abundant heterointerfaces, unique yolk-shell structure and increased carbon interlayer spacing in Co1−xS-CoS2/carbon nanofibers (Co1−xS-CoS2/CNF), the reaction kinetics and the diffusion rate of Na+ have been significantly improved. As expected, the as-prepared Co1−xS-CoS2/CNF anode exhibits extraordinary rate performance (495.3 mAh/g at 10 A/g) and cycling stability (380.1 mAh/g at 5 A/g after 5,000 cycles), outperforming most reported TMS materials. A sodium-ion full cell with a Co1−xS-CoS2/CNF anode and Na3V2(PO4)3 cathode also show excellent electrochemical performance. The electrochemical reaction mechanism of Co1−xS-CoS2/CNF is revealed by ex-situ XRD and ex-situ HRTEM. Hence, it is hoped that our work can provide constructive ideas for the design of electronic or morphological structures for building high-performance energy storage materials.