Synergistic Bimetallic Interaction and Regulated Void Size in Isocubanite CuFe₂S₃ Enables UltraFast and Durable Sodium Storage

Iron-based bimetallic sulfides featuring dual redox-active centers and abundant reserves are gradually emerging as potential anodes for advanced sodium-ion batteries (SIBs). However, they still suffer from capacity fading and inferior rate capability due to volumetric expansion and inadequate conductivity. Herein, isocubanite CuFe2S3 nanoparticles embedded in N,S-codoped porous carbon fiber (CuFe2S3@C) have been constructed by electrospinning and subsequent sulfuration processes using polystyrene (PS) nanospheres as the absorbent and void regulator. Precise regulation of the void structure in composite materials is achieved by the selection of PS nanospheres. Furthermore, the introduction of Cu atoms leads to enhanced conductivity and a low Na+ migration barrier in CuFe2S3@C. Synchrotron radiation measurements provide compelling evidence for the enhanced strength of the Fe–S bond, facilitating the maintenance of structural stability. Additionally, its structural reversibility is supported by the consistent 57Fe Mössbauer spectra of the pristine and cycled states. Consequently, the optimized CuFe2S3@C exhibits outstanding cyclic stability (delivering a reversible capacity of 360 mAh g–1 after 800 cycles at 5 A g–1, with almost a 100% capacity retention) and impressive rate capability (252 mAh g–1 at 30 A g–1). When paired with a commercial Na3V2(PO4)3 cathode, the coin full cell yields an 86.5% capacity retention after 200 cycles. This work encourages the development of bimetallic sulfide anodes with excellent sodium storage performance.