Epitaxial nanofiber separator enabling folding-resistant coaxial fiber-supercapacitor module

Mechanically strengthened fiber-supercapacitors (FSCs) are urgently needed in wearable electronics to adapt frequent severe deformations. However, current bendable FSCs using gel electrolyte as separator are easily short-circuited under harsh folding due to the limited stretchability of gel polymer. Herein, a folding-resistant coaxial FSC, for the first time, was fabricated through in-situ electrospinning polyacrylonitrile (PAN) nanofibers as separator on fiber electrode, thus effectively avoiding the short-circuit risk under highly localized stretching, compressing and folding. Moreover, such “epitaxial growth” ultrathin (∼ 1 µm) PAN separator possesses high porosity, favorable strength and seamless contact with fiber electrode, enabling fast ion transport and decreased internal resistance of FSCs. Coupled with robust polyaniline/carbon nanotube (PANI/CNT) composite as model electrodes, our fabricated coaxial symmetric FSC exhibits outstanding structural folding-resistance without electrochemical failure. Amazingly, such in-situ encapsulated separator technique makes it easier to assemble multi-layer PANI/CNT electrode in series or parallel. As-assembled coaxial integrated series or parallel device keeps almost same volume as single device, but delivers higher output voltage or capacitance; and simultaneously remains excellent foldable property, showing great practical potential.