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electrode materials, the fabricated SCs exhibited a specific capacitance of 60.435 mF cm⁻² at the scan rate of 10 mV s⁻¹ and a high energy density of 18.88 μW h cm⁻² with an extended potential window of 1.5 V. The specific areal capacitance curves in Figure 2.5e showed a slightly decrease when the SC was stretched to 20%. Despite the limited stretchability of the coaxial SCs, the electrochemical performance was much improved, paving a great support to the real application of the stretchable SCs.

      2.2.2 Planar Stretchable SCs

      Stretchable 2D planar SCs with excellent properties of small size, low weight, excellent lifespan, high security and easy integration have become a preferred choice as energy storage to power the wearable electronics [37, 61–63]. There are two main categories including layer by layer sandwich structure and micro supercapacitors (MSCs). The fabrication method of the 2D planar stretchable SCs is similar to 1D fiber shaped SCs. One is via embedding rigid independent devices to the stretchable substrate or establishing serpentine interconnects between rigid devices to realize stretchability. Another one is replacing the rigid unit by stretchable component. In this section, we will introduce the typical fabrication method reported during the last few years.

      2.2.2.1 Fabrication of the Stretchable Planar SCs with Sandwich Structure

Sandwiched planar SCs is the most developed and widely applied structure in the field of the SCs. The main issues that impede the successful fabrication of stretchable SCs is how to make a tight connection between electrode materials and elastic substrate. As early as 2009, Yu et al. proposed a strategy via directly transferring the single‐walled carbon nanotube (SWCNT) film to pre‐strained PDMS substrate [64]. It's worth mentioning that the PDMS substrate was treated by UV light to forming a strong bong with SWCNT film. The fabrication process was presented in Figure 2.6a. Figure 2.6b showed the SEM images of the SWCNT film with buckled microstructures, which directly contribute to the stretchability of the fabricated SCs. Figure 2.6c displayed the CV profiles of the stretchable SCs measured at 30% strain. From the CV curves, we can see that the electrochemical performance of the stretchable SCs remains unchanged even under 30% applied tensile strain. But the utilization of a polymeric separator restricted its stretchability. To overcome this problem, Niu et al. reported stretchable SCs based on periodically sinusoidal oriented SWNT film without separator and liquid electrolyte using the same pre‐strained‐buckled‐release method, which showed increased stretchability of over 120%.

      Most of the deformable substrate used in the field of stretchable SCs is PDMS. In 2014, Xie et al. reported a flat Ni foam based stretchable all‐solid‐state SC with wavy shaped polyaniline (PANI)/graphene electrode [65]. Figure 2.6d showed the schematics of the fabrication process for fabricating the PANI/graphene electrodes based stretchable SCs. First, a flat Ni foam with a thickness of 200 mm was manually made into a wavy shape, next, the porous graphene was synthesized on the buckled Ni foam via atmospheric pressure chemical vapor deposition (CVD). Then the graphene coated Ni foam was put in a solution of 3 M HCl to etch nickel foam to obtain wavy‐shaped graphene film. In order to improve the electrochemical performance of the SCs, the PANI was deposited on the wavy shaped graphene film. Finally, two PANI covered graphene films with PVA/H₃PO₄ wall were encapsulated into Elastic substrate (Ecoflex). Figure 2.6e depicted the CV curves of the stretchable SCs at different tensile strains. The initial specific capacitances calculated from CV curves were 261 F g⁻¹. It can be seen that no obvious change appeared when the SC was stretched to 30%. Moreover, the stretching cycle tests revealed that the SC maintained high mechanical strength and stability over 100 cycles.

      2.2.2.2 Omnidirectionally Stretchable Planar SCs

From the aforementioned stretchable actions, it can be concluded that stretchability can be realized without affecting on the electrochemical performances of the SCs. There are even some operations with enhanced electrochemical performance when the SCs devices was stretched due to the more contact between electrode and gel electrolyte under stretching. Unfortunately, these actions can only be stretched along one direction, hence, if it is possible to make SCs isotropic stretchable, the electrochemical performance, such as specific capacitance, cycle stability etc. also could be improved in a certain degree.

      

Figure 2.6 (a) Fabrication process of the stretchable SCs by buckling electrode materials on an elastomeric PDMS substrate. (b) SEM image of a buckled CNT macro film. (c) CV profiles of the stretchable SCs measured at 30% strain.

      Source: Reproduced with permission [64]. © 2009, Wiley‐VCH.

      (d) Schematics of the stretchable SCs fabrication. (e) CV curves of the stretchable SCs at different tensile strains.

      Source: Reproduced with permission [65]. © 2014, The Royal Society of Chemistry.

In 2016, Yu et al. designed a novel isotropic wavy shaped CNT film electrode based omnidirectionally stretchable SCs, as shown in Figure 2.7 [66]. Figure 2.7a showed the fabrication process. In detail, the PDMS elastic film was uniformly pre‐strained in all direction, and then CNT film was transformed to the PDMS substrate, finally, after relaxation, the omnidirectionally stretchable SCs were assembled. The SEM images of side view of buckled CNT film on silicon rubber substrate were presented in Figure 2.7b. The buckled CNT film and tight connection will benefit the stretchability. Figure 2.7c demonstrated the excellent isotropic stretchability of the buckled CNT film. The resistance variation after 10 000 uniaxially stretching−releasing cycles was also measured (Figure 2.7d), which showed a little increase (< 3%) at tensile strains of 200%. Figure 2.7e displayed the CV curves of the fabricated SCs with various deformable states. No significant change was observed in the CVs with 200% applied omnidirectional strains. Moreover, the specific area capacitance was improved from 1160.43 to 1230.61 mF cm⁻² during uniaxial, biaxial, and omnidirectional elongations, which showed wide potential application in stretchable electronics.

Figure 2.7

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