Flexible Supercapacitors. Группа авторов
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[29] with properties of large strain and free shape, which make stretchable devices work excellent under various deformation. Self‐healability as an attractive high elasticity has been included into stretchable substrate, which could get the wearable devices back to life while the devices inevitably encounter malfunction during their lifetime. Several kinds of self‐healable substrates like polyvinyl alcohol (PVA) [30], polyacrylic acid (PAA) [31], polyacrylamide (PAM) [32] have been synthesized in recent years.
All‐solid‐state gel electrolytes are one of the essential components for stretchable SCs in design of fully wearable energy storage devices, which could protect SCs from the risks of liquid leakage and simplify the device configurations of the SCs by removing the extra separator and substrates of the devices. On the basis of solvent type, gel electrolytes are generally considered to fall into the below two categories: hydrogel electrolyte and organic ions/solvent‐based gel electrolyte. The ionic conductivity of hydrogel electrolyte can reach 10−4 S cm−1, up to 10−1 S cm−1 [2]. Organic gel electrolyte exhibits excellent stability in air and enhanced electrochemical performance by adding organic ions to electrolyte, which is a unique way to improve the electrochemical properties of SCs devices.
To date, many achievements have been made to fabricate stretchable SCs that include both 1D fiber SCs, 2D planar SCs and 3D structured SCs [33, 34]. The fiber shaped energy devices have been woven into flexible energy textiles while and the planar devices have been proved that they could be attached to the human body. Figure 2.1 presents the typical 1D, 2D, and 3D stretchable SCs and their application in wearable electronics, as well as the multifunctional stretchable SCs contain self‐healable SCs, compressible SCs, and their integration with other wearable devices.
In this chapter, we focus on the recent progress in stretchable SCs and their potential application in wearable electronics. First, the main approaches to assemble stretchable SCs consist of both 1D fiber and 2D planar devices are presented. Then, we describe the main electrochemical and mechanical performances in the field of the stretchable SCs. The multifunctional SCs such as self‐healable SCs, compressible SCs and the self‐powered integrated system are also highlighted. Finally, we discuss the existing challenges and future development trends for stretchable SCs.
2.2 Fabrication of Stretchable Supercapacitor
As previously mentioned, viewed from device dimension, SCs have three main categories, 1 D fiber SCs, 2D planar, and 3D SCs. Here, the devices, structure design and strategies for making SCs stretchable in three dimensions are summarized. These approaches can be used to fabricate stretchable devices, which have potential applications beyond stretchable energy storage, including all‐in‐one stretchable integrated system, where all unites with the same substrate, high stretchability and general applicability are desirable.
2.2.1 Structures of Stretchable Fiber‐Shaped SCs
Recently, 1D SCs, also known as fiber, wire, or yarn‐shaped SCs with advantages of light weight and easy integration have received significant attention as one ideal form for wearable electronics because they can be weaved into textiles or smart clothes [43–46]. 1D fiber electrode held together in different configurations can form several kinds of fiber shaped SCs, including parallel, twisted, and coaxial structure. Parallel SCs refer to two fiber electrodes constructed side by side, these two fiber electrodes can be attached on the same two fiber substrates or both sides of one substrate in a parallel state [47]. Twisted configuration is a structure of two intertwining fiber electrodes, separated by all‐solid‐state gel electrolyte in the middle, which is the most popular structures in fabrication of fiber SCs [48]. The layout of two electrodes of coaxial SCs can be geometrically viewed as a column situated within a cylinder sharing the same center axis, the inter electrode and outer electrode are separated by gel electrolyte or separator, showing a core–sheath configuration [44, 49, 50].
Figure 2.1 Summary of stretchable SCs and their application in integrated system 1D fiber SCs: Twisted SCs,
Source: Reproduced with permission [35]. © 2014, Wiley‐VCH.
Coaxial SCs,
Source: Reproduced with permission [36]. © 2013, Wiley‐VCH.
2D planar SCs, the individual elements are:
Source: Reproduced with permission [37]. © 2013, Wiley‐VCH. Reproduced with permission [38]. © 2013, American Chemical Society. Reproduced with permission [39]. © 2017, Wiley‐VCH.
3D SCs,
Source: Reproduced with permission [40]. © 2016, American Chemical Society.
Multifunctional: self‐healable SCs,
Source: Reproduced with permission [31]. © 2015, Nature Publishing Group.
Compressible SCs,
Source: Reproduced with permission [41]. © 2015, WILEY‐VCH.
Integrated system,
Source: Reproduced with permission [101]. © 2018, Elsevier Ltd.
By design of different structure, the electrochemical performance of fiber SCs can be improved. For example, Peng's group [51] made a comparative study to analyze electrochemical performance of coaxial and twisted SCs, in detail, carbon nanotube (CNT) sheet was scrolled on the another CNT sheets with a gel electrolyte coat to fabricate coaxial devices and two CNT sheets fiber electrodes were intertwined to obtain twisted SCs, the electrochemical tests showed that coaxial SCs exhibit larger specific capacitance of 59 F/g than that of twisted SCs (4.5 F g−1) because the distance between positive and negative electrodes of the coaxial SCs is more closer and the utilization of materials is more effective compared to the twisted one. In order to understand how the configuration effect on the electrochemical performance of fiber shaped SCs, our group built a theoretical model using ANSYS Maxwell software [52]. The simulated parameters used in simulation system were particularly organized to ensure the unity of volume, superficial area, materials, relative permittivity, and positive/negative voltage between coaxial and twisted geometries. At first, two electrode models were carried out with the same electrode material and electrolyte parameters, the voltage of electrode was set from −0.4 V to 0.4 V. The analysis results on electric field under the same conductions were displayed, as shown in Figure 2.2. Figure 2.2a and b depicted the schematic diagram of the coaxial and twisted fiber SCs used in the theoretical model. Figure 2.2c and d displayed the corresponding electrostatic potential distribution around the two fiber SCs. Compared to the scattered, asymmetrical, and sigmoid potential distribution of the twisted devices, the potential distribution of coaxial SCs is more homogeneous, leading to the better electric charge‐transfer effect and lower contact resistance. Figure 2.2e and f showed the energy distribution simulation results of the twisted and coaxial devices.
Figure 2.2 Structure and voltage, energy distribution of 1D fiber SCs: twisted and coaxial.