Nanoconfining ultrafine heterostructured sulfides in carbon nanofibers enabling ultrastable and fast sodium storage

Transition metal sulfides (TMSs) are considered as promising anodes for sodium ion batteries (SIBs) due to their high theoretical specific capacity and low redox potential. Unfortunately, poor structural stability and sluggish kinetics of the sulfides result in an unsatisfactory cycling stability and rate performance. Herein, the ultrafine Co9S8/WS2@C particles embedded in carbon nanofibers (CNF) can be obtained via nanoconfinement strategy coupled with electrospinning technique, and the as-synthesized Co9S8/WS2@C@CNF realizes the achievement of superb rate performance (378.8 mA h g−1 at 2 A g−1), and stable structural stability (92.1% capacity retention after 700 cycles at 2 A g−1). Specifically, these outstanding electrochemical properties are closely related with the introduction of β-cyclodextrin (β-CD) during the synthesis process, in which the β-CD can act as the nanoreactor and the activator for confining ultrafine nanoparticles and generating porous structure. The combining of large surface area, ultrafine particles, and rich heterointerfaces in Co9S8/WS2@C@CNF can effectively reduce the Na+ diffusion length and promote reaction kinetics. The impressive electrochemical character of Co9S8/WS2@C@CNF anode is further confirmed in full-cell Na-ion batteries, which also displays ultra-long-life span (92.9% capacity retention after 500 cycles). It should be noted that the unique nanoreactor with nanoconfinement function can be adapted to the construction of other metal sulfide electrodes for building ultra-stable energy-storage devices.