A flexible and free-standing FeS/sulfurized polyacrylonitrile hybrid anode material for high-rate sodium-ion storage

Sodium-ion based energy storage systems have attracted extensive attention due to the similarities in the mechanism of operation with lithium-ion batteries along with the additional benefit of low cost and high abundance of sodium resources. Iron sulfide-based electrodes that operate via conversion mechanism have shown ample potential for high energy sodium-ion storage. However, the problems related with tremendous volume changes and the dissolution of sodium polysulfides in the electrolyte deteriorate the cycle life and limit their application in sodium-ion batteries (SIBs). Herein, a hybrid anode material, FeS/SPAN-HNF, with iron sulfide (FeS) nanoparticles decorated in a sulfurized polyacrylonitrile (SPAN) fiber matrix is demonstrated as flexible and free-standing anode material for high-rate SIBs. Unlike previous strategies in which FeS is encapsulated in an electrochemically inactive carbon matrix, this study utilizes SPAN, an electrochemically active material, as a dual functional matrix that can efficiently buffer volume expansion and sulfur dissolution of FeS nanoparticles as well as provide significant capacity improvement. The as-designed electrode is self-standing and flexible, without current collectors, binders or additional conductive agents, thus rendering enhanced practical capacity and energy density. This electrode showed a high reversible capacity of 782.8 mAh g−1 at 200 mA g−1 with excellent high rate capability, maintaining 327.5 mAh g−1 after 500 cycles at 5 A g−1, emphasizing promising prospects for the development of flexible and high energy density SIBs.