Structural evolution and sodium storage behaviors of hollow vs solid hard carbon nanofibers for high-rate sodium ion batteries

Hard carbon (HC) is a promising anode for sodium-ion batteries (SIBs) due to its low cost and favorable electrochemical properties. However, its rate capability remains limited by sluggish ion transport and structural constraints. Herein, solid and hollow hard‑carbon nanofibers (HC-Fs and HC-HFs) are fabricated via uniaxial and coaxial electrospinning, respectively, to elucidate the role of architecture in sodium-storage behavior. The hollow structure endows HC-HFs with enlarged surface area, hierarchical porosity, and shortened ion/electron transport pathways, enabling enhanced reaction kinetics. Operando and in situ characterizations, including X-ray diffraction, Raman spectroscopy, and electrochemical impedance spectroscopy coupled with distribution of relaxation time (DRT) analysis, reveal that the hollow architecture facilitates Na + adsorption, intercalation, and diffusion while promoting the formation of a stable and conductive solid-electrolyte interphase. As a result, HC-HFs exhibit reduced interfacial and diffusion resistances compared with HC-Fs. Benefiting from a three-dimensional interconnected and binder-free framework, HC-HFs deliver a high reversible capacity of 206.7 mAh g −1 at 2120 mA g −1 , demonstrating excellent high-rate performance. This work provides fundamental insights into structure-kinetics relationships and offers an effective strategy for designing advanced carbon anodes for fast-charging SIBs.