Abstract
1D yarn/fi ber supercapacitors are more attractive power sources than conventional 3D, 2D foam/fi lm-type when they are used for miniaturized electronic devices, textile electronics, and implantable medical devices [ 1 ] because their small volume and high fl exibility enable them to be easily integrated into the devices with a tiny size and various shapes. Nevertheless, most fi ber-based supercapacitors (FBSs) simply possess the fl exibility with limited tensile strain and stretchability [ 2–16 ] because they are mostly based on nonstretchable electrodes such as carbon nanotube (CNT) spun yarn, [ 2–7 ] graphene fi ber, [ 8–12 ] carbon fi ber, [ 13,14 ] and metal wire. [ 15 ] This lack of stretchability of these FBSs leads to limitation for more advanced utilization, e.g., as a power source for artifi cial muscles [ 17 ] or wearable devices that are exposed to high strain, especially in the joint part. [ 18 ] In recent years, several strategies have been reported to incorporate stretchability into FBSs: CNT sheet nanomembranes were wound onto elastic fi bers to make highly stretchable FBSs [ 19–21 ] or CNT fi ber electrodes were attached onto prestrained elastic fi bers to form a microbuckling structure for FBSs. [ 22,23 ] Although these FBSs exhibited stable stretchability and energy storage performances, they still suffer from low electrochemical energy storage performance because the additional core fi bers used as elastomeric substrates do not participate in electrochemical energy storage reactions but merely provide stretchability, resulting in low gravimetric and volumetric energy storage performances of the total devices. [ 24 ] To solve these problems, novel spring-like stretchable CNT fi bers were proposed as supercapacitor electrodes. [ 24 ] It is notable that stretchability could be achieved for the fi rst time in FBSs by using the spring CNT fi ber without the help of any elastomeric substrate. However, the spring-based supercapacitors still showed limited capacitances because their active material was based on pristine CNT, totally depending on the electrochemical double layer capacitance (EDLC) energy storage mechanism. All things considered, achieving FBSs possessing not only stretchability but also excellent specifi c capacitances remains challenging.