One step preparation and excellent performance of CNT yarn based flexible micro lithium ion batteries

Flexible lithium ion batteries (LIBs) are promising energy storage devices that can facilitate the advancement of wearable and flexible electronics. The main drawbacks of flexible LIB development are their low energy densities, poor mechanical strengths, and lack of flexibility. In this paper, we developed a facile method to prepare flexible and freestanding LIBs by modifying the CNT yarn (CNTY) synthesis. Iron catalysts in CNTYs were directly converted to electrochemically-active Fe2O3 nanoparticles featuring homogeneous distribution without agglomeration, monodisperse, and tight adhesion to CNT. This unique microstructure in the electrochemically activated CNTY (A-CNTY) electrode contributed to a good electrochemical performance (capacity, cyclability, and rate capability) of the resulting CNTY when evaluated as an anode material for LIB. We also fabricated a flexible fiber battery prototype composed of A-CNTY anode and commercially available lithium iron phosphate coated CNTY cathode with a gel electrolyte. The fabricated batteries are 30 cm-long with a diameter of a few hundred micrometers. In addition, our CNTY-based flexible LIBs are able to endure deformation without any substrates due to the good mechanical properties of CNTY. Our fiber type LIBs were able to successfully power various LED lights even during deformation.