FeS2 nanoparticles embedded in N/S co-doped porous carbon fibers as anode for sodium-ion batteries

[Display omitted] •FeS2@CF-NS were synthesized by electrostatic spinning with subsequent calcination.•The carbon backbone buffers the volume expansion.•The doped N, S and defect sites facilitate fast and stable Na+/e− exchange.•The FeS2 nanoparticles and FeS2 nanoflakes shorten the Na+ diffusion distance.•Capacitive Na+ storage mechanism contributes to the rate performance. FeS2 is a promising electrode material for sodium ion batteries (SIBs) because of its high theoretical capacity, rich reserves, and eco-friendly nature. In this study, N and S doped (N, S-co-doped) carbon fibers (CFs) encapsulated FeS2 nanoparticles (5–12 nm) and adherent FeS2 nanoflakes (denoted as FeS2@CF-NS), were synthesized by electrostatic spinning and subsequent thermal treatment. In this structure, the FeS2 nanoparticles and the FeS2 nanoflakes shorten the Na+ diffusion distance; the N, S co-doping and defect-rich sites in the carbon fibers accelerate the Na+/e− transmission and buffer the volume expansion during the Na-FeS2 conversion reaction. These merits synergistically contribute to the notable sodium storage performance of FeS2@CF-NS. As anode for Na-ion half batteries, the FeS2@CF-NS exhibits high capacity (637.1 mAh/g at 1 A/g after 400 cycles) and excellent rate capacity (431.1 mAh/g at 5 A/g). Kinetic analysis confirms that this composite structure stimulates the pseudocapacitance Na+ storage mechanism and enables a capacitive contribution ratio as high as 92.7% with respect to the total capacity. In combination with Na3V2(PO4)3-C cathode, the FeS2@CF-NS also achieves remarkably high specific capacity (561.1 mAh/g at 1 A/g after 500 cycles) and stable cyclability (338.6 mAh/g at 5 A/g after 5000 cycles) in full cells.