Figure 2.4 (a) Schematic illustration of fabricating twisted SCs by wrapping aligned CNT sheet around a pre‐stretched elastic wire. (b) Top‐view and cross section SEM images of the fiber SCs. (c) Changes in the normalized capacitance under stretching.

      Source: Reproduced with permission [58]. © 2014, Wiley‐VCH.

      (d) Schematics of the fabrication procedures for stretchable fiber SCs by twisting the devices. (e) Optical microscopy images the fiber devices. (f) Specific capacitance of the fiber SCs under stretching.

      Source: Reproduced with permission [35]. © 2014, Wiley‐VCH.

      Another group of stretchable twisted SC have no ability of stretchability itself, with the whole devices twined around the elastic substrate. For example, Xu et al. proposed a fabrication method by using spandex fiber as the substrate (Figure 2.4d) [35]. Specifically, a fiber‐shaped SC was fabricated by twisting two CNT based fiber electrodes. Then, next, spandex fiber was pre‐stretched to 100% with a PDMS coat to prevent influence from acidic H₂SO₄‐PVA gel electrolyte. And then the fiber‐shaped SCs were “glued” onto the spandex fiber. When the substrate was released, the SCs with stretchability over 100% were accomplished. Figure 2.4e showed the photography of the serpentiform stretchable SCs. A slightly enlarged of the CV loop areas suggested the enhanced capacitance when the SC was stretched to a large strain of up to 100%. Moreover, the specific capacitance of the fiber SCs under stretching (Figure 2.4f) also surpassed those of the initial unbuckled SCs after 20 cycles of deep stretching. The outstanding stretchability can contribute to the buckled structures formed after releasing the strain. This fabrication method could make most of the twisted SCs stretchable, but the devices and the stretchable substrate are two separate parts, which is unfavorable to keeping the electrochemical performances under repeated deformation. Nevertheless, more advanced technologies for solving the above‐mentioned problems are demanded imminently to realize its practical application in wearable electronics.

      2.2.1.3 Fabrication of Stretchable Coaxial SCs

Among the three structures of fiber SCs, the fabrication of stretchable coaxial SCs is the most complicated, but this configuration makes the device more integrated than the parallel and twisted structures [59]. The biggest difficulty lies in the fabrication of sheath/shell‐like electrode (outer electrode). To date, many efforts have been devoted to the development of novel coaxial SCs. For example, Lee et.al proposed an assembly method by wrapping separator and carbon nanofiber (CNF) film on the surface of the core fiber electrode, but it makes the contact resistance large and the electrode materials waste [44]. In contrast to this structure, Peng's group reported a stretchable coaxial SCs that fabricate layer by layer, from inside to outside, in sequence of inner electrode, gel electrolyte and outer electrolyte without separator, as shown in Figure 2.5a and b [36]. Figure 2.5a showed the schematic illustration of the corresponding fabrication process. An elastic fiber was used as stretchable core fiber substrate. To start with, a coat of PVA‐H₃PO₄ gel electrolyte was made on the surface of the elastic fiber. Next, the aligned CNT sheets were wrapped around the gel electrolyte coted elastic fiber as the inner electrode. Followed by coating the second layer of electrolyte as separator and wrapping with another CNT sheet as the outer electrode. Finally, the third layer of gel electrolyte was spread around the outer electrode. The fabricated SCs exhibited a specific capacitance of 20 F g⁻¹, mass energy of 0.515 W h kg⁻¹and power densities of 421 W kg⁻¹. Figure 2.5b presented the CV profiles of the stretchable coaxial SCs with increasing strains. The overall stretchability could reach as high as 100% with a good specific capacitance retention. The specific capacitance was remained above 90% after 1000 charge‐discharge cycles when applied a strain of 75%, showing a promising stability under deformation. Although they achieved the goal of produce stretchable coaxial SCs, unfortunately, reports so far on high performance have been very limited. Great efforts should also be focused on promoting the electrochemical properties of stretchable coaxial devices.