Fiber-based solid-state electrolytes: Multiscale characterization and design for flexible energy devices

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Fiber-based solid-state electrolytes (FBSSEs) have emerged as promising electrolyte platforms for flexible energy storage devices, driven by the increasing demand for safe, lightweight, and deformable power sources. Compared with conventional film-type and bulk solid-state electrolytes, FBSSEs offer continuous ion-transport pathways, mechanical adaptability, and tailorable interfacial structures, enabling improved safety, flexibility, and electrochemical stability. Nevertheless, their practical application remains challenged by limited ionic conductivity, unstable electrode/electrolyte interfaces, unclear ion-transport behavior under deformation, and the lack of standardized evaluation protocols. This review summarizes recent progress in FBSSEs for flexible energy storage. The material characteristics and design principles of polymer fibers, inorganic ceramic fibers, and organic/inorganic composite fibers are first discussed. Representative fabrication strategies, including electrospinning, dry/wet spinning, and 3D printing, are then reviewed, with emphasis on fibrous architecture regulation and ion-conduction network construction. Their applications in flexible lithium-ion batteries, lithium-sulfur batteries, metal-air batteries, and supercapacitors are highlighted. Advanced characterization techniques, including solid-state nuclear magnetic resonance, neutron scattering, and synchrotron X-ray methods, are also discussed for elucidating ion migration, interfacial evolution, and structural dynamics. Finally, challenges and future perspectives are outlined, focusing on multiscale structural design, mechanism-guided characterization, standardized testing, and scalable manufacturing.