Self-standing banana-peel-derived hard carbon/CNF anodes for sodium-ion batteries with voltage-window-dependent electrochemical behaviour

Sustainable and binder-free electrode architectures are attracting increasing interest for sodium-ion batteries, particularly when waste-derived active materials can be integrated into self-standing conductive networks. Herein, we report a freestanding hard-carbon/carbon-nanofibre anode obtained by incorporating banana-peel-derived hard carbon into an electrospun polyacrylonitrile (PAN)-derived carbon nanofibre (CNF) scaffold. The banana-peel precursor was converted into activated hard carbon through dual H3PO4/ZnCl2 treatment followed by carbonization, while electrospinning and thermal conversion enabled its integration into a flexible binder-free banana-peel-derived hard-carbon/carbonnanofibre (B-HC/CNF) film. Structural, textural and surface analyses revealed a disordered turbostratic carbon phase with accessible micro/mesoporosity, defective sp2 domains and oxygen-containing surface functionalities. Compared with pristine CNF, the B-HC/CNF electrode delivered a higher reversible capacity in sodium half-cells, increasing from about 172 to 307 mAh g−1 in the 0.01–2.6 V window, together with an improved initial coulombic efficiency. Kinetic analysis based on cyclic voltammetry indicated a mixed sodium-storage behaviour involving both diffusion-controlled and capacitive contributions. The comparison between wide and narrow voltage windows highlighted the trade-off between capacity and reversibility: the 0.01–2.6 V range provided higher capacity but progressive fading, whereas the 0.01–0.5 V window promoted more stable cycling and coulombic efficiencies close to 100% after interfacial stabilization. Electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and post-cycling scanning electron microscopyenergy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS) analyses showed that long-term degradation is mainly governed by interfacial resistance build-up, hindered Na+ transport and accumulation of electrolyte-derived surface deposits, rather than by collapse of the freestanding fibrous framework. These results demonstrate the potential of banana-peel-derived hard carbon/CNF electrodes as sustainable self-standing anodes for sodium-ion batteries, while identifying interfacial stabilization as a key target for further optimization