A braided flexible Zn–MnO2 yarn battery based on cobweb-like carbonized polypyrrole modified carbon fiber electrodes

Flexible zinc ion batteries (ZIBs) are promising candidates for energy storage devices because of high safety and low cost. Carbon fibers (CFs) are ideal current collectors for ZIBs due to their good flexibility, electrical conductivity and corrosion resistance. Herein, a novel CFs-based current collector is proposed. At first, the inertness and non-polarity of CFs were significantly modified by activating and etching to obtain a multi-layered porous structure. Thereafter, carbonized polypyrrole (CPPy) nanowire conductive networks were built up on etched carbon fibers (ECFs) by electrochemical in situ growth and carbonization. Based on the CPPy network, MnO2 nanoflowers were grown firmly by the hydrothermal method to construct multi-layer MnO2@CPPy@ECFs composite electrode. The multi-layered 3D CPPy conductive network is rooted in MnO2, providing a dedicated transmission path for both electrons and ions during electrochemical redox reactions. The MnO2@CPPy@ECFs electrode had an initial capacity of 346.1 mA h·g−1 at 0.1 A g−1 and 252.8 mA h·g−1 after 800 cycles. In addition, the integrated flexible Zn–MnO2 2D fabric battery with MnO2@CPPy@ECFs fabric cathode shows a specific capacity of 254.7 mA h·g−1 at 0.1 A g−1. Finally, a flexible Zn–MnO2 yarn battery with high flexibility, shape adaptability, structural integrity and good mechanical stability was fabricated by the 2D braid method. The initial specific capacity of the flexible Zn–MnO2 yarn battery is 111.7 mA h·g−1 at 0.1C, which also presents a remarkable volumetric energy density of 30.5 mW h cm−3. This work opens up a new pathway for the rational design of flexible electrodes for wearable ZIBs.