Mechanically Robust and Chemically Stable Separator Membrane Constituted of Electrospun Halloysite-Integrated Core–Shell Nanofibers for Sodium-Ion Batteries

The emerging demand for efficient and sustainable energy storage systems has driven significant interest in sodium-ion batteries (SIBs) as an economic substitute for lithium-ion batteries (LIBs). Among the key components of SIBs, the separator serves a crucial role in governing electrochemical performance and ensuring operational safety under various working conditions. This study explores the fabrication of advanced nanofiber separators using coaxial electrospinning, focusing on a core–shell composite structure composed of polyacrylonitrile (PAN) as the core of the nanofibers and halloysite nanotubes (HNTs) integrated polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) as the shell of the nanofibers. The effect of this architecture on the structural integrity and electrochemical performance is systematically investigated. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) confirm the optimized morphology, phase, and chemical bonding in the fabricated composite nanofiber separator. Outcomes show that the morphology of the HNT integrated PVDF-HFP/PAN coaxial composite separator (CCS) is uniform, and most of the fibers have a diameter range of 200–400 nm. These nanoscale features of CCS contribute a commendable set of properties, including high mechanical strength (24 MPa), high thermal stability (170 °C), high porosity (74%), and electrolyte uptake (325%). Electrochemical evaluations reveal superior ionic conductivity (1.86 mS cm–1), transference number (0.63), and a broad electrochemical stability window (5 V). The battery cell assembled with a CCS showed excellent performance, delivering the maximum discharge capacity of 159.58 mA h g–1 at 0.1 C rate and retaining 87.32% after 100 charge–discharge cycles at 0.5 C rate. This research demonstrates the potential of a unique core–shell nanofibrous structure to deliver outstanding electrochemical performance, paving the way for its application in next-generation SIBs.