Mechanically flexible V ₃ S ₄ @carbon composite fiber as a high‐capacity and fast‐charging anode for sodium‐ion capacitors

Hybrid Na‐ion capacitors (NICs) have received considerable interests owing to their low‐cost, high‐safety, and rapidly charging energy‐storage characteristics. The NICs are composed of a capacitor‐type cathode and a battery‐type anode. The major challenge for NICs is to search for suitable electrode materials to overcome the sluggish diffusion of Na + in the anode. Herein, ultrafine vanadium sulfide is encapsulated in carbon fiber (V 3 S 4 @CNF) as a self‐supported electrode by electrospinning and in situ sulfurization. The carbon cladding and one‐dimensional (1D) nanofiber network‐like structure could alleviate the volume expansion of V 3 S 4 during Na + de‐/intercalation process. Consequently, the V 3 S 4 @CNF anode exhibited a pseudocapacitive sodium storage in terms of large Na + ‐storage capacity (476 mAh·g −1 at 0.1 A·g −1 ), high‐rate capability (290 mAh·g −1 at 20.0 A·g −1 ) and excellent cycling stability (95% capacity retention for 1500 cycles at 2.0 A·g −1 ) in Na half‐cells. By employing V 3 S 4 @CNF as the anode and the activated carbon (AC) cathode, the as‐assembled NICs could deliver a high energy density of 110 Wh·kg −1 at a power density of 200 W·kg −1 . Even at a high power of 10,000 W·kg −1 , the specific energy is still up to 42 Wh·kg −1 .