Unlocking Native Defects in Carbon Fibers via Helical Strain Engineering for High‐Performance Stretchable Supercapacitors

Carbon fiber's (CF) inherent chemical inertness limits its use in wearable energy textiles. This study introduces an innovative helical strain engineering strategy to activate CF's latent electrochemical activity by controllably generating cross‐sectional defects. This low‐energy, scalable mechanical process induces programmable microfracture, creating high‐performance, burr‐enriched CF yarns (CFYs) for stretchable supercapacitors. Controlled twisting coupled with pre‐stretched polyurethane and polyester fibers formed native microdefects with exposed active edge sites and an interconnected 3D conductive network. The optimized CFY4 substrate exhibited ultra‐low resistivity (0.22 Ω·cm), 40% reversible stretchability, and a burr density of 6.51%. Additionally, in situ polymerization of polypyrrole (PPy) on CFY4 produced a pseudocapacitive electrode (CFY4@PPy) with a mass‐specific capacitance of 141.41 F g −1 at 0.3 A g −1 , which was 2.5 times higher compared to pristine CF@PPy. The assembled symmetric yarn‐based supercapacitor achieved a wide voltage window of 2.5 V, high energy density of 53.94 Wh kg −1 at 500 W kg −1 , cyclic voltammetry curve stability under 30% tensile conditions. Successful demonstrations powering LEDs, sensors, and wearables confirm practical viability. This work provides a new paradigm in carbon material engineering while demonstrating how controlled mechanical deformation can unlock the hidden electrochemical potential of CF for next‐generation wearable energy textiles.