A Low‐Permeability and Flexible Polymer Tube for Long‐Life Fiber Lithium‐Ion Batteries

Abstract Fiber lithium‐ion batteries (FLIBs) hold great promise for powering wearable electronics. However, their practical application is hindered by limited cycle and calendar life, primarily due to the loss of active Li caused by water vapor permeation through the encapsulation layer. To address this challenge, a low‐permeability and high‐flexibility tetrafluoroethylene hexafluoropropylene copolymer (FEP) tube is presented to continuously encapsulate FLIBs by melting extrusion method. Owing to the inherent hydrophobicity of fluorine resins and appropriate crystallinity of the polymer matrix, FEP tubes exhibited significantly low vapor permeability, with a water vapor transmittance rate (WVTR) of 0.3 mg·day −1 ·pkg −1 , 15 times lower than that of the nylon 12 tubes (4.6 mg·day −1 ·pkg −1 ). Leveraging the low permeability and elastic modulus of FEP tubes, FLIBs demonstrate a capacity retention of 80.05% after 180 cycles and exceptional flexibility with a capacity retention of 98.32% after 10 000 bending cycles, showcasing superior performance compared to the conventional polymer tubes (for example, the capacity of PP‐FLIBs declined by 20.68% after 30 cycles). This work presents a general and efficient strategy for continuously encapsulating FLIBs, effectively extending their cycle and calendar lifetime of FLIBs, thereby enhancing their practical viability for wearable electronic applications.