Electrospun Core–Shell Carbon Nanofibers as Free-Standing Anode Materials for Sodium-Ion Batteries

The development of wearable devices requires flexible batteries that can be bent and folded. However, deficiencies in material flexibility, conductivity, and other aspects can affect the performance of a flexible electrode. One-dimensional nanofibers possess high specific surface area, high conductivity, and a 3D network structure that enables them to buffer stress and strain. Consequently, they hold significant potential in the field of electrochemical energy storage as flexible electrode materials. In this study, we utilized polyvinylpyrrolidone (PVP) as a template to form a supramolecular polymer (PNDS) through hydrogen bonding between 1,4,5,8-naphthalene tetracarboxylic acid (NTCA) and 3,3′-diaminobenzidine (DAB) monomers. Through core–shell electrospinning and carbonization, PNDS precursors were used to prepare polybis(benzimidazobenzophenanthroline-dione) (BBB)-based carbon nanofibers featuring a core–shell structure. The BBB polymer, featuring a continuous aromatic ring structure, undergoes conversion into a highly graphitic carbon skeleton upon carbonization at 1000 °C. This transformation enhances the conductivity of flexible electrodes, improves the current collection effect under high currents, and ensures stability in the charge–discharge cycle. The iron-containing polymers within the shell layer ultimately transform into iron oxide and iron carbide nanoparticles encapsulated within the carbon fibers, compensating for the lower specific capacity characteristic of pure carbon materials. Serving as a SIB flexible anode, the specific capacity can achieve 250 mAh g–1 after 950 cycles at 0.2 A g–1, with negligible attenuation throughout the cycling process. This study demonstrates that BBB-based carbon nanofibers featuring core–shell structures exhibit excellent electrochemical performance while retaining flexibility, presenting clear advantages over traditional electrodes characterized by complicated processes and limited active substance content.