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2.12 (a) Schematics of a 3D stretchable SC. (b) Optical images of the CNT film being gradually stretched along the z axis. (c) GCD curves with increasing strains from 0% to 16% along the x or y axis.

      Source: Reproduced with permission [74]. © 2016, The Royal Society of Chemistry.

      (d) Schematics for the assembling process of stretchable SCs. (e) Digital images of 3D SCs under different strain tests. (f) Lighting test driven by tailored paper‐cutting SC.

      Source: Reproduced with permission [41]. © 2017, Wiley‐VCH.

      By the kirigami technique, the SCs can be designed with stretchability, as shown in Figure 2.12d [41]. In comparison with prestrain‐release based stretchable SCs, the form and shape of editable are more free. Cellular structure, pyramid structure, living‐hinge structure etc. can be easily obtained and the stretchable direction are not restricted. Traditional sandwiched SCs always use gel electrolyte as the separator, which caused the weak intermolecular interaction thus cannot effectively serve as a separator to prevent short circuit of SCs during editing process. Therefore, the authors employed nanocellulose fibers as separator and PVA/H₃PO₄ as gel electrolyte. MnO₂ nanowires@CNT served as electrode materials. Figure 2.12e displayed the photography of fabricated honeycomb shaped SCs with increasing strain varying from 0% to 810%. The fabricated honeycomb shaped SCs exhibited a specific capacitance of 227.2 mF cm⁻² and no degradation of electrochemical performance when applied a strain of 500%, providing an excellent mechanical stability of the editable SCs. In addition, nearly 98% of initial capacitance of the honeycomb shaped editable SCs interlocked by PU fibers was remained even after 10 000 stretch‐and‐release cycles under reversible 400% tensile strain. As mentioned above, the editable SCs endow advantages of tunable voltage by different interconnection in parallel and/or series. As a proof of concept, four SCs were connected in series and tailored into delicate and artistic patterns to power a LED (Figure 2.12f), suggesting that editable SCs can realize more complicated and free patterns with high stretchability.

2.3 Multifunctional Supercapacitor

      With the development of the highly stretchable SC, its application was expanded to many areas, including portable, wearable energy storage, electronic skins, sensors, detectors, implantable medical devices, which also raise new demands to the energy storage, such as compressible, self‐healable etc. In recent years, many attempts have been made to incorporate such functions into stretchable energy storage.

      2.3.1 Compressible SCs

Compressible SCs have emerged as a new branch of stretchable SCs. Compared to stretchable electronics, compressible SCs endow the large levels of strain without sacrificing basic electrochemical performance. Traditional compressible SCs usually use graphene, SWCNTs or their composite foam as electrode materials, which can't be compressed into one unit due to the liquid electrolyte and incompressible encapsulation. To address these problems, Niu et.al proposed a nanostructured composite sponge based all‐solid‐state SCs with a remarkable compression tolerance [75]. The PANI@SWCNTs sponge electrode materials were synthesized as follows (Figure 2.13a): the sponge was cut into required shapes and then repeatedly dipped with SWCNTs solution. After drying, the electrochemical deposition process was performed to coat a PANI nanomaterials on the surface of the SWCNTs covered sponge. The mass loading of SWCNT and PANI was 2.2 and 4.1 mg cm⁻², respectively.

Figure 2.13b showed the schematic diagram of the cross‐sectional scheme of the fabricated sandwiched compressible SCs. Two patterned Au film on poly(ethyleneterephthalate) (PET) served as the current collector attaching to the outer‐top and outer‐bottom side of sponge electrodes, separated by filter paper and PVA/H₂SO₄ gel electrolyte. The single device during stress‐release cycle was illustrated in Figure 2.13c. Figure 2.13d displayed the CV curves under different compressible strain. The specific capacitance of PANI@SWCNT sponge electrodes based all‐solid‐state SC calculated from CV curves was 216 F g⁻¹. Almost 97% of capacitance retention was maintained even under 60% compressible strain. Furthermore, four SCs (Figure 2.13e and f) were connected in series to improve the overall output potential, which was powerful enough to light up an LED. We can see the brightness of the LED under compressible strain (Figure 2.13g and h) did not show any noticeable change with a compressing releasing cycle, demonstrating the excellent stability of the compressible all‐solid‐state SCs, which will pave the way for practical applications of SCs in the field of compressible energy storage devices to fit the demands of the compression‐tolerant electronics.

Figure 2.13 (a) Schematic diagram of synthesizing PANI@SWCNTs sponge composite electrodes. (b) Cross‐sectional scheme of the fabricated PANI@SWCNTs sponge based SCs. (c) Real‐time optical images of the SCs under compressing. (d) CV curves of the compressible SCs with increasing strains from 0% to 60%. (e) Photography of the patterned d Au current collector on PET plate. (f–h) Lighting test driven by four SCs showing the compressing and recovering process.

      Source: Reproduced with permission [75]. © 2015, Wiley‐VCH.

      2.3.2 Self‐Healable SCs

Self‐healability as a remarkable function that helps SCs from complete damage and then prolong the lifespan of the energy storage has been introduced into stretchable SC systems [76]. Similarly, self‐healable SCs possess two main categories, including fiber shaped energy storage and planar SCs. 1D self‐healable SCs devices have received tremendous attention in recent years as they can be woven into textiles and fit the curved surface of the human body as well as easily integration with portable and wearable electronics. Until now, many 1D fiber shaped self‐healable SCs have been designed. For example, our group proposed a self‐healable fiber SCs by twisting two NiCo₂O₄ electrode covered PVA/KOH hydrogel [77]. During the damaging‐healing cycles, 82.19% of capacity was remained after four cycles, achieving the goal of the reactivated work once the devices was damaged. Zhi's group reported a twisted 1D self‐healable SCs with self‐healable PU shell [28]. In their work, magnetic Fe₃O₄ materials electrode were directly grown on the bare yarn via hydrothermal and annealed process, then a 2 μm thickness of polypyrrole (PPy) film was deposited on the surface of Fe₃O₄ nanomaterials. The schematic illustration of the self‐healing process and self‐healing mechanism were displayed in Figure 2.14a and b. It can be concluded that the excellent self‐healing properties derive from the synergistic effects between the self‐healing PU shell and the magnetic Fe₃O₄ electrodes. In detail, the strong intermolecular hydrogen bonds of PU shell are reversible and could reestablish when the broken components are brought into contact. Besides, the magnetic Fe₃O₄

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