A novel cord-shaped supercapacitor with high stretchability

Abstract

Cord-shaped stretchable supercapacitors have received much attention due to their high stretchability and three-dimensional flexibility. For coaxial supercapacitors, lower capacitive performance and the inextensibility of the separation layer have been the important factors hindering further developments. In this work, we developed an integration technology to fabricate multiple yarn-electrodes by a layer-to-layer braiding. Between the inner and outer electrodes there is a braided elastic separator by braiding multiple spandex yarns. The braided separator features not only a reliable separation of the inner and outer electrodes, but also a high stretchability and high porosity. The cord-shaped supercapacitor shows an excellent capacitive performance of 60.1 mF/cm² with energy density of 5.4 μWh/cm², power density of 0.52 mW/cm² and extremely high capacitance retention of 99.8% under the strain of 100%, demonstrating the application value of this stretchable energy storage device.

Keywords

stretchability cord-shaped coaxial braided electrode separator supercapacitor

Stretchable electronics is an emerging field in large-area electronics wherein circuits can be built or embedded in stretchable substrates, which have the capacity to accommodate large strains without significant degradation in their electronic performance.^1–4 It has burgeoning potential in biomedical devices, wearable electronics, artificial skin incorporating sensor abilities and so on.^5–8 Currently, some devices, such as light-emitting diodes (LEDs) and display panels, retain their function when stained up to 100%.^9,10 As essential components of stretchable electronics, energy storage units, such as batteries and supercapacitors, should be capable of accommodating large strain while retaining the performance to match these highly stretchable devices.^11–13

Recently, attention has been paid to fiber-shaped stretchable supercapacitors. For example, Yang et al.¹⁴ demonstrated a fiber-shaped supercapacitor by wrapping aligned carbon nanotube (CNT) sheets on an elastic fiber. The high stretchability and specific capacitance (19.2 F/g at 0.1 A/g) were both achieved by means of a coaxial structure. As a special configuration, the fiber-shaped supercapacitors can achieve a high level of strain and three-dimensional flexibility, which enables them to be woven into a cloth, or be integrated into microelectronics, realizing the value of the energy supply.

However, the capacitance of a single fiber or yarn-electrode is not enough to meet the requirement of the energy supply. The high internal electric resistance brought by increasing the length of the fiber electrode¹⁵ and the low flexibility by integrating multiple fiber electrodes together¹⁶ greatly limits the development of fiber-shaped stretchable supercapacitors. More importantly, it is critical for stretchable coaxial supercapacitors to achieve a high stretchability while avoiding the contact between the inner and outer electrodes leading to a short circuit, which easily happens because of the non-stretchability of the separation layer.¹⁷ However, few studies have focused on the stretchable separation membrane.

In this study, we developed a stretchable coaxial cord-shaped supercapacitor by means of layer-by-layer braiding. The inner and outer electrodes were coaxially braided with six coated yarns and the yarns were coated with electrochemical active materials, along an elastic yarn as the core, to form an integration of multiple yarn-electrodes in the circumferential direction. The stretchable separation layer was fabricated by 12 braiding spandex filament yarns between the inner and outer electrodes. The developed cord-shaped supercapacitor in this work can achieve more than 100% strain while maintaining more than 99.8% areal specific capacitance of 60.1 mF/cm² (volume specific capacitance 1.5 F/cm³; mass capacitance 31.02 F/g), demonstrating an excellent application value.

Experimental details

Materials

Commercial polylactic acid (PLA) filament yarn of 150D/72f was purchased from Benhui Textile Co., Ltd. Pyrrole monomer, anthraquinone-2-sulfonic acid (AQSA), FeCl₃·6H₂O, polyvinyl alcohol (PVA) and phosphoric acid (H₃PO₄) were purchased from Sinopharm Chemical Reagent Co., Ltd. Spandex monofilament yarns of 420, 860 and 1220 D were purchased from INVISTA Fiber Co., Ltd.

Surface modification of PLA filament yarn

Graphene oxide (GO) was home-made according to the modified Hummers' method.¹⁸ Briefly, H₂SO₄/H₃PO₄ (9:1) was added to a mixture of graphite flake and KMnO₄. Then, the mixture was heated to 50℃ and stirred for 12 h. The reaction was cooled down to the room temperature and 30% H₂O₂ was added. After that, the GO solution was filtered and centrifuged. Lastly, GO aqueous solution of 2 mg/mL was made under ultrasonic treatment.

PLA filament yarn was rinsed with acetone to remove impurities. A solution of GO was adsorbed on the PLA filament by the dipping and coating process. After repeating the process, the GO/PLA (GO@PLA) filament was obtained. The GO@PLA filament was immersed into a 20 ml aqueous solution containing pyrrole (0.02 mol/L) and AQSA (0.01 mol/L). Then, 20 ml FeCl₃ (0.18 mol/L) was dropwise added to initiate polymerization. After a chemical in situ polymerization in an ice bath for 8 h, the GO/polypyrrole (PPy)/PLA (GO/PPy@PLA) filament yarn was obtained. The modified PLA yarn showed a capacitance of 158.8 mF/cm² and conductivity of 652 S/m. The yarn coated with the electrochemical active materials (hereafter was named “modified PLA yarn”) was then used in the following process to fabricate coaxial electrodes.

The reason for coating GO on the PLA filament prior to the polymerization of PPy was to make PPy grow more uniformly on the yarn surface. The process was inspired by previous studies^19,20 indicating an effect of improvement in coating evenness of PPy. The effect of evenness improvement was also observed in our preliminary experiment. The reason for the improvement of PPy coating evenness was attributed to the conjugation of GO with the pyrrole monomers through π–π stacking interaction.

Fabrication of stretchable coaxial cord-shaped electrodes and separator

The samples were braided in a 12-spindle braiding machine (Type C-12, Shanghai Hakao Thread and Narrow Fabric Machinery Co., Ltd, China). The take-up speed of the braiding samples was so adjusted that the braiding angle was kept at 30°. The fabrication process of a stretchable cord-shaped electrode is shown in Figure 1(a). The inner electrode was braided by using six modified PLA yarns along a core of 1220 D spandex monofilament yarn, which was pre-stretched by 200% before braiding. After stretching recovery, a stretchable cord-shaped inner electrode was obtained with more than 100% stretchability. A series of braiding samples was made with different numbers (from four to eight) of PLA yarns to evaluate its effect on the stretchability and capacitance of the developed supercapacitor.

Figure 1.

Fabrication process of (a) the inner electrode and (b) the separator and outer electrode. PLA: polylactic acid.

The stretchable separator was fabricated by braiding 12 spandex monofilament yarns on the surface of the inner electrode. To examine the influence of the diameter of the spandex monofilament yarns on the mechanical and electrochemical properties of the cord-shaped supercapacitor, a series of braiding samples with stretchable separators was fabricated by using spandex monofilament yarns of different diameters (420/860/1220 D). The tubular spandex braided layer outside the inner electrode could achieve a full coverage for the inner electrode to avoid a contact short circuit with the outer electrode immediately outside the braiding separator. The braided separator provided a high stretchability and a porous structure, allowing a full impregnation of the electrolyte inside the coaxial structure.

After braiding the spandex separator, the outer electrode was fabricated by braiding the modified PLA yarns, of which the number and the braiding angle were the same as those in the inner electrode, on the surface of the separation layer, as shown in Figure 1(b), to finish the process. After the injection of PVA and H₃PO₄ as the electrolyte, the fabrication of the coaxial cord-shaped supercapacitor was completed. More than 100% strain was achieved after a preliminary stretch test.

Characterization

To characterize the capacitive properties of the developed cord-shaped electrodes and to clarify the effect of structural parameters of the supercapacitor on the energy storage performance, electrochemical measurements were carried out using a three- or two-electrode test cell on a CHI 660D electrochemical workstation (Shanghai Chenguang Co., China) at room temperature. Cyclic voltammetry (CV) was conducted in the range of 0–0.8 V with an incremental sweep rate of 5 mV/s. Galvanostatic charge–discharge (GCD) properties were measured at different current densities with a cutoff voltage of 0–0.8 V. In addition, the capacitance retention (the specific capacitance of the developed supercapacitor at different tensile strains as the percentage of that in the initial state) of the developed supercapacitor was tested under different strains and stretch cycles by a JF-9003 tensile tester (Dongguan Jianfeng Instrument Co., Ltd, China) with a crosshead speed of 100 mm/min and maximum strain of 100%.

Results and discussion

Electrochemical properties of the braided electrode

Before electrochemical property characterization, the tensile properties of the modified PLA yarns were tested in a yarn tester (Type YG061FQ, Laizhou Electron Instrument Co., Ltd). Figure 2 shows the testing results, in which the modified PLA has a high tensile strength.

Figure 2.

Tensile properties of polylactic acid (PLA) yarn before and after the modification.

To choose a suitable number of braiding yarns for the inner and outer electrodes, a series of samples with a varying number of modified PLA yarns was fabricated and electrochemical performances were measured. Figure 3(a) shows the CV curves of the electrodes braided with different numbers of the modified PLA yarns. Each modified PLA yarn provides the capacitance as a single unit of the braided electrode. The braiding process was in fact combining together a number of units of the PLA yarns. Therefore, the area enclosed by the CV curves increased with the number of braiding modified PLA yarns. This means that the electrode with more modified PLA yarns could provide larger capacitance of the braided electrode.

Figure 3.

(a) Cyclic voltammetry curves of the braided electrodes with different numbers of braiding yarns. (b) Effects of the number of braiding yarns on the specific capacitance and the maximum elastic stain of the electrodes.

However, it is shown in Figure 3(b) that the benefit of increasing the capacitance of the braided electrode by increasing the number of braiding yarns is gradually overtaken by the reduction of the maximum elastic strain when the number of braiding yarns is increased. The maximum elastic strain here refers to the maximum strain at which the samples retain elasticity during tensile deformation. It was noticed that, with the increasing number of braided yarns, the maximum elastic strain of the braided electrode decreased rapidly, and the tensile strain even dropped to less than 100% when the braiding yarn number was more than six. A compromise was then made between the stretchability and the capacitance of the braided electrode by choosing six braiding yarns each for the inner and outer electrodes as the optimal number in the present study. According to the CV curve in Figure 3(a), the areal capacitance of the six-yarn braided electrode was 198.7 mF/cm² (7.23 F/cm³; 68.7 mF/cm; 93.8 F/g).

Electrochemical properties of the cord-shaped supercapacitor

To discuss the influence of some key structural parameters, such as the length of the braided electrodes and the diameter of the spandex monofilament in the electrochemical properties of the developed supercapacitor, electrochemical tests were carried on a CHI 660D electrochemical workstation in the two-electrode mode. From the CV curves in Figure 4, it can be seen that the braided electrodes are somehow polarized. Two reasons lead to such a phenomenon. (i) A longer length of the braided electrode would lead to a greater electrode resistance. It is shown in Figure 4(a) that, by reducing the electrode length from 4 to 2 cm, the electrochemical performance is effectively improved. (ii) The braided separation layer made it difficult for the electrolyte to impregnate into the inner electrode when the porosity of the separation layer was too low. To examine the influence of the porosity of the separator, other conditions remained the same. Spandex monofilaments with three different finenesses, 1220 D (diameter 0.5 mm), 860 D (diameter 0.3 mm) and 420 D (diameter 0.1 mm), were chosen to braid the separator. The results in Figure 4(b) show that the energy storage performance could be improved by using a finer diameter spandex monofilament to achieve more porosity of the separator. According to the comparison of the area enclosed by the CV curves, in the present study, the optimal length of the developed cord-shaped supercapacitor is 20 mm, and the diameter of the spandex monofilament for the braiding separator is 0.1 mm (420 D).

Figure 4.

Cyclic voltammetry (CV) curves and bar charts of the area enclosed by the CV curves for the cord-shaped supercapacitor: (a) with different lengths of electrodes; (b) with different diameters of the spandex yarns braiding the separator.

The CV curves at different scanning rates and charging–discharging curves at different current densities of the cord-shaped supercapacitor with the chosen structural parameters (i.e. 2 cm length of the electrode and 0.1 mm diameter spandex yarns for the braiding separator) are shown in Figure 5. The CV curves form a completely closed pattern, implying that this structure has a reversible charge–discharge capacitance performance. The GCD curves maintain approximately symmetrical triangles at different current densities, indicating a great coulombic efficiency, which is as high as 85% with the charge–discharge current of 0.25 mA. The areal capacitance of the developed supercapacitor is 60.1 mF/cm² (1.5 F/cm³; 31.02 F/g); the areal energy density is 5.4 μWh/cm² (0.13 mWh/cm³; 2.8 Wh/kg); the maximum power density is 0.52 mW/cm² (12.34 mW/cm³) at the current 0.5 mA. Therefore, it is possible to use the chosen structural parameters to fabricate the coaxial cord-shaped supercapacitor, achieving the integration of multiple energy storage yarns to gain an excellent capacitive performance.

Figure 5.

(a) Cyclic voltammetry curves at different scan rates. (b) Charging–discharging curves at different current densities of the developed supercapacitor.

According to the Nyquist plot in Figure 6, it can be worked out²¹ that the equivalent series resistance (ESR), the intercept of the high-frequency region with the real axis, is rather low as 0.65 KΩ and the charge transfer resistance, the small semi-circle region, is about 35 Ω, indicating a rapid charge transport. The high slope of the curve indicates easy ion diffusion at the low-frequency region.

Figure 6.

Nyquist plots.

Figure 7 shows the Ragone plot for energy density and power density of the developed supercapacitor. It reaches a very high level of cord-shaped power density of 0.52 mW/cm² (12.34 mW/cm³; 184 W/kg) and energy density of 5.4 μWh/cm² (0.13 mWh/cm³; 2.8 Wh/kg). In Figure 7, a comparison was made between the developed cord-shaped supercapacitor and some typical stretchable fiber/yarn supercapacitors reported in the literature. For those comparison counterparts, there were two typical approaches to obtain the stretchability. One was to wind a solid-state fiber electrode, such as graphene, into helical coils.^22–24 The other was by spinning technology to obtain stretchable CNT fibers as the electrode.²⁵ By comparison, the cord-shaped supercapacitor developed in the present study shows a much higher energy because it was able to integrate multiple yarn-electrodes to enhance the performance.

Figure 7.

Ragone plot of the supercapacitors.

Capacitance retention after stretching and cyclic stretching

Figure 8(a) shows the capacitance retention of the developed cord-shaped supercapacitor (length: 2 cm; diameter: 1.6 mm) under 0–500 cycles of dynamic stretching and releasing (DSR) with 100% strain, when the speed of the crosshead was 100 mm/min. Although the capacitance retention was reduced with the increasing DSR cycles, 88.5% of the retention could still be achieved after stretching for 500 cycles. Figure 8(b) shows that the developed cord-shaped supercapacitor possesses excellent capacitance retention under different strains. High capacitance retention of 99.8% can be achieved under the strain of 100%. When the developed cord-shaped supercapacitor was stretched, the deformation of the inner and outer electrodes could be maintained by changing the braiding angles of the modified PLA yarns without stretching the active material on the surface of PLA yarns, which guaranteed an extremely high capacitance retention.

Figure 8.

Capacitance retention line charts under (a) 0–500 dynamic stretching and releasing (DSR) cycles and (b) different strains.

After the injection of the electrolyte, the developed cord-shaped supercapacitor could light up a LED light (1.35 V, 6 mA, 8.1 mW), as shown in Figure 9.

Figure 9.

Two supercapacitors in series to light up a light-emitting diode light.

Conclusions

In this study, we fabricated a stretchable coaxial cord-shaped supercapacitor by means of a layer-by-layer braiding process. Each of the inner and outer electrodes was braided with six modified PLA yarns along a pre-stretched core yarn of spandex monofilament. The electrode could achieve 100% strain and a high capacitance of 68.7 mF/cm. Between the inner and outer electrodes there was a stretchable separation membrane fabricated by braiding 12 spandex yarns on the surface of the inner electrode, which featured high stretchability and porosity. By injecting the electrolyte between the inner and outer electrodes, the assembly of the cord-shaped supercapacitor was completed. In this way, multiple yarn-electrodes could be integrated by the braiding process, which endowed the cord-shaped supercapacitor with an excellent areal capacitive performance of 60.1 mF/cm² (1.5 F/cm³; 31.02 F/g), the energy density of 5.4 μWh/cm² (0.13 mWh/cm³; 2.8 Wh/kg) and power density of 0.52 mW/cm² (12.34 mW/cm³). A very high capacitance retention of 99.8% could be achieved under the strain of 100%, demonstrating the application value of this stretchable cord-shaped supercapacitor.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

References

Kettlgruber

Kaltenbrunner

Siket

, et al. Intrinsically stretchable and rechargeable batteries for self-powered stretchable electronics. J Mater Chem A 2013; 18: 5505–5508.

, et al. Stretchable fiber supercapacitors with high volumetric performance based on buckled MnO2/oxidized carbon nanotube fiber electrodes. Small 2017; 12: 1602994.

Shi

Yang

Song

, et al. Stretchable wire-shaped supercapacitors with high energy density for size-adjustable wearable electronics. Chem Eng J 2017; 322: 538–545.

Dong

Wang

Deng

, et al. A highly stretchable and washable all-yarn-based self-charging knitting power textile composed of fiber triboelectric nanogenerators and supercapacitors. ACS Nano 2017; 9: 9490–9499.

Khang

Jiang

Huang

, et al. A stretchable form of single-crystal silicon for high-performance electronics on rubber substrates. Science 2006; 5758: 208–212.

Masarapu

Rong

, et al. Stretchable supercapacitors based on buckled single-walled carbon-nanotube macrofilms. Adv Mater 2010; 47: 4793–4797.

Zhu

. Buckling of aligned carbon nanotubes as stretchable conductors: a new manufacturing strategy. Adv Mater 2012; 24: 1073.

Liu

Song

Kong

, et al. Flexible and stretchable energy storage: recent advances and future perspectives. Adv Mater 2017; 29: 34.

Krull

De Graff

, et al. Stretchable inorganic-semiconductor electronic systems. Adv Mater 2011; 23: 2933–2936.

10.

Yun

Niu

, et al. Compliant silver nanowire-polymer composite electrodes for bistable large strain actuation. Adv Mater 2012; 24: 1321–1327.

11.

Lee

Kim

. Stretchable carbon nanotube/ion-gel supercapacitors with high durability realized through interfacial microroughness. ACS Appl Mater Interfaces 2014; 6: 13578–13586.

12.

Gaikwad

Zamarayeva

Rousseau

, et al. Highly stretchable alkaline batteries based on an embedded conductive fabric. Adv Mater 2012; 24: 5071–5076.

13.

Yuan

Han

Xiao

, et al. Flexible electrochemical capacitors based on polypyrrole/carbon fibers via chemical polymerization of pyrrole vapor. Appl Surf Sci 2016; 377: 274–282.

14.

Yang

Deng

Chen

, et al. A highly stretchable, fiber-shaped supercapacitor. Angewandte Chemie 2013; 125: 13695–13699.

15.

Liu

Yan

, et al. Wearable energy-dense and power-dense supercapacitor yarns enabled by scalable graphene-metallic textile composite electrodes. Nat Commun 2015; 6: 9.

16.

Chen

Hao

Peng

, et al. High-performance, stretchable, wire-shaped supercapacitors. Angew Chem Int Ed Engl 2015; 54: 618–622.

17.

Huang

Wang

Zheng

. Textile-based electrochemical energy storage devices. Adv Energ Mater 2016; 6: 1600783.

18.

Danlela

Marcano, Dmltry

, et al. Improved synthesis of graphene oxide. ACS Nano 2010; 4: 4806–4614.

19.

Zhu

Zhai

Wen

, et al. Graphene oxide/polypyrrole nanocomposites: one-step electrochemical doping, coating and synergistic effect for energy storage. J Mater Chem 2012; 22: 6300–6306.

20.

Raccichini

Varzi

Passerini

, et al. The role of graphene for electrochemical energy storage. Nat Mater J 2015; 14: 271–279.

21.

Liu

Song

Kong

, et al. Flexible and stretchable energy storage: recent advances and future perspectives. Adv Mater 2017; 29: 1603436.

22.

Meng

Zhao

, et al. All-graphene core-sheath microfibers for all-solid-state, stretchable fibriform supercapacitors and wearable electronic textiles. Adv Mater 2013; 25: 2326–2331.

23.

Cao

, et al. Carbon nanotube fiber based stretchable wire-shaped supercapacitors. Adv Energ Mater 2014; 4: 6.

24.

Cai

, et al. Fiber supercapacitors utilizing pen ink for flexible/wearable energy storage. Adv Mater 2012; 24: 5713–5718.

25.

Chen

Qiu

Guan

, et al. Novel electric double-layer capacitor with a coaxial fiber structure. Adv Mater 2013; 25: 6436–6441.