Densifiable Ink Extrusion for Roll‐To‐Roll Fiber Lithium‐Ion Batteries with Ultra‐High Linear and Volumetric Energy Densities

Abstract Conventional bulky and rigid planar architecting power systems are difficult to satisfy the growing demand for wearable applications. 1D fiber batteries bearing appealing features of miniaturization, adaptability, and weavability represent a promising solution, yet challenges remain pertaining to energy density and scalability. Herein, an ingenious densifiable functional ink is invented to fabricate scalable, flexible, and high‐mass‐loading fiber lithium‐ion batteries (LIBs) by adopting a fast ink‐extrusion technology. In the formulated ink, pyrrole‐modified reduced graphene oxide is elaborately introduced and exerts multiple influences; it not only assembles carbon nanotubes and poly(vinylidene fluoride‐co‐hexafluoropropylene) to compose a sturdy, conductive, and agglomeration‐free 3D network that realizes an ultra‐high content (75 wt%) of the active materials and endows the electrode excellent flexibility but also serves as a capillary densification inducer, encouraging an extremely large linear mass loading (1.01 mg cm ⁻¹ per fiber) and packing density (782.1 mg cm ⁻³ ). As a result, the assembled fiber LIBs deliver impressive linear and volumetric energy densities with superb mechanical compliance, demonstrating the best performance among all the reported extruded fiber batteries. This work highlights a highly effective and facile approach to fabricate high‐performance fiber energy storage devices for future practical wearable applications.