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 electrodes can also electrically reconnected when the devices were cut into pieces. Figure 2.14c showed the CV curves of the fabricated SCs after several healing cycles. The specific capacitance of the self‐healing device obtained from the CV curve was 61.4 mF cm−2 at a scan rate of 10 mV s−1. We can see even after four cutting–healing cycles, 71.8% of the capacitance retention was observed, providing an excellent self‐healing performance. This work may encourage the design and fabrication of the 1D fiber shaped self‐healable energy storage and wearable electronics.
Self‐healable 2D planar SCs are another attractive energy storage because they possess the advantage of 2D devices like small size, low weight, ease of handing in appearance as well as the ultralong lifespan of the stretchable electronics. As a typical example, Huang et.al fabricated a 2D planar self‐healable SC based on PAA dual cross linked by hydrogen bonding and vinyl hybrid silica nanoparticles (VSNPs) [31]. Figure 2.14d illustrated the schematics of the fabrication process of the highly stretchable and self‐healable SCs. As for electrode materials, CNT papers were synthesized by CVD, and then deposited with the PPy, which were attached on the both side of the self‐healable VSNPs‐PAA gel electrolyte‐based film to prepare a self‐healable planar SCs. Figure 2.14e showed the demonstration of and ionic conductivity of the self‐healed substrate. The wound in the VSNPs‐PAA film could be autonomously repaired via the intermolecular hydrogen bonds among the cross‐linked polymer chains on the VSNPs in 10 mins under the ambient condition, which has no effect on the ionic conductivity and mechanical properties of the VSNPs‐PAA film. The CV curves with different cut‐healing times were depicted in the Figure 2.14f. The fabricated self‐healable SCs exhibited a specific capacitance of 61.4 mF cm−2 at a scan rate of 10 mV s−1, which was kept unchanged even after four healing cycles.
2.3.3 Stretchable Integrated System
Stretchable integrated systems have demonstrated a great real‐word application in wearable electronics, such as electronic skin that monitor heart rate, wrist pulse, body temperature, voice etc., robots that move as instructed, bio‐medicine carrier that reflect signal, while leaving people's normal activities of daily life unaffected [78–82]. These integrated systems that compose of the energy harvester, energy storage, and the functional sensing/detecting units are usually connected on the same deformable substrate [83–87]. In comparison with Li‐ion battery, fuel cells, and other types of battery, SCs have gained extensive attention out of security consideration. To date, several types of integrated devices toward the direction of portable and wearable electronics, like energy storage‐sensor unit and self‐powering system have been successfully fabricated. In harmony with the configuration of the stretchable SCs, the fiber substrate‐based 1D integrated device, PDMS based 2D integrated system and elastic materials‐based 3D devices have shown a rapid development momentum in the field of deformable integrated system. For integration of different functional devices with stretchability, there are three vital factors need to be considered: (i) coordinate work, each unit or functional partition could realize
