Two-step synthesis of millimeter-scale flexible tubular supercapacitors.

Flexible supercapacitors have been demonstrated to be ideal energy storage devices owing to their lightweight and flexible nature and their high power density. However, conventional film-shaped devices struggle to meet the requirements of application in complicated situations, including medical instruments and wearable electronics. Here we report a hollow-structured flexible tubular supercapacitor prepared from a scalable method with the same diameter as electric wires. This new supercapacitor design allows for a large specific capacitance of 102 F g at a current density of 1 A g with excellent air-working stability over 10,000 cycles. It also shows a high energy density of 14.2 Wh kg with good rate capability even at a current density of 10 A g, which is superior to commercial devices (3-10 Wh kg). Moreover, the device delivers a stable energy storage capacity when encountering different flexible conditions, such as elongated, tangled and bent states, showing wide potentials in flexible and even wearable applications. Especially, it retains stable specific capacitance even after 500 bending cycles with a bending angle of 180°. The two-step fabrication method of these flexible tubular supercapacitors may allow for possible mass production, as they could be easily integrated with other functional components, and used in realistic scenarios that conventional film devices struggle to realize.