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Flexible Energy-Storage Devices: Design Consideration and Recent Progress Xianfu Wang, Xihong Lu, Bin Liu, Di Chen, Yexiang Tong,* and Guozhen Shen* devices, containing lithium-ion batteries and supercapacitors, are usually too heavy, rigid, and bulky to match the particular requirements of flexible electronics.[5,6] Therefore, the trend in the next generation of energy-storage-device development is to realize light, flexible, and small units with shape-conformability, aesthetic diversity, and excellent mechanical properties. The key challenges in achieving flexible energy-storage devices are the selection and design of bendable current collectors with good mechanical properties and the fabrication of flexible electrode materials with a high capacity and excellent conductivity. To fulfill and ameliorate flexible energy-storage devices, such as flexible lithium-ion batteries and flexible supercapacitors, tremendous effort has been made in recent years. Figure 1 shows several typical examples of designed flexible energy-storage devices and their potential applications in flexible electronics, printable electronics, wearable electronics, and integrated systems. This review focuses on the recent development on flexible energy-storage devices, including flexible lithium-ion batteries (LIBs) and flexible supercapacitors (SCs). In the first part, we review the latest successful examples of flexible LIBs based on conductive paper, three-dimensional electrodes, solid-state electrolytes, and novel structures, along with their technological innovations and challenges. In the following section, a detailed overview of the recent progress regarding flexible SCs based on carbon materials and other composites coupled with flexible micro-supercapacitors are given, combined with discussion of the future challenges and opportunities for the fabrication of the state-of-the-art flexible supercapacitors. Finally, we briefly introduce some of the latest achievements in the recently highlighted integrated system based on energy-storage devices.

Flexible energy-storage devices are attracting increasing attention as they show unique promising advantages, such as flexibility, shape diversity, light weight, and so on; these properties enable applications in portable, flexible, and even wearable electronic devices, including soft electronic products, roll-up displays, and wearable devices. Consequently, considerable effort has been made in recent years to fulfill the requirements of future flexible energy-storage devices, and much progress has been witnessed. This review describes the most recent advances in flexible energy-storage devices, including flexible lithium-ion batteries and flexible supercapacitors. The latest successful examples in flexible lithium-ion batteries and their technological innovations and challenges are reviewed first. This is followed by a detailed overview of the recent progress in flexible supercapacitors based on carbon materials and a number of composites and flexible micro-supercapacitors. Some of the latest achievements regarding interesting integrated energystorage systems are also reviewed. Further research direction is also proposed to surpass existing technological bottle-necks and realize idealized flexible energy-storage devices.

1. Introduction Recently, accompanied with the development of flexible electronics, great interest has been aroused in flexible/bendable electronic equipment, such as wearable devices, rollup displays and bendable mobile phones.[1–4] However, current energy-storage

X. Wang,[+] B. Liu, Prof. G. Shen State Key Laboratory for Superlattices and Microstructures Institution of Semiconductors Chinese Academy of Science Beijing 100083, PR China E-mail: [email protected] X. Wang, B. Liu, Prof. D. Chen Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic Information Huazhong University of Science and Technology (HUST) Wuhan 430074, PR China Dr. X. Lu,[+] Prof. Y. Tong KLGHEI of Environment and Energy Chemistry MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry School of Chemistry and Chemical Engineering Sun Yat-sen University Guangzhou 510275, PR China E-mail: [email protected] [+]These authors contributed equally to this work

DOI: 10.1002/adma.201400910

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2. Flexible Lithium-ion Batteries The design of soft portable electronic equipment, such as rollup displays, smart cards, wireless sensors, and wearable devices, requires flexible, lightweight, and environmentally friendly lithium-ion batteries (LIBs) with a high energy density, long cycle, and excellent rate capability.[7–11] Several routes toward the development of flexible batteries, such as polymer batteries,[12,13] Cellulose-based batteries,[14] paperbased batteries,[15–18] and soft packing batteries,[19,20] are being

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Prof. Yexiang Tong received his BS in general chemistry in 1985, MS in physical chemistry in 1988, and PhD in organic chemistry in 1999 from Sun Yat-Sen University. He joined Sun Yat-Sen University as an Assistant Professor of Chemistry in 1988. His current research focuses on the electrochemical synthesis of alloys, intermetallic compounds and metal oxide nanomaterials, and investigation of their applications for energy conversion and storage. Prof. Guozhen Shen received his Ph.D degree in 2003 from University of Science and Technology of China. He joined the Institute of Semiconductors, Chinese Academy of Sciences as a Professor in 2013. His current research focuses on flexible electronics and printable electronics, including transistors, photodetectors, sensors, and flexible energy-storage devices.

Figure 1. Examples of several typical flexible energy-storage devices and their potential applications in next-generation flexible electronics.

explored. It has been found that the flexibility of functional devices are largely determined by the electrodes or the current collectors.[21] Thus, to fully realize flexible LIBs, soft electrodeactive materials and bendable current collectors are strongly demanded. The softness and bendability of the nanostructured inorganic materials render them as the potential candidates electrodes of the flexible LIBs. Moreover, the electrochemical reactions of batteries are commonly determined by electron and ion transport at the surface of the electrodes. By using nanostructured materials, the rates of the electron and ion transport can be highly increased due to the reduced size shortening the pathway.[9] This section highlights the most interesting recent studies on nanostructured electrode materials developed for flexible LIBs.

2.1. Paper Lithium-Ion Batteries Paper electronics have been a research goal of next-generation electronics and have attracted much attention recently. To meet the demands of paper electronics, paper-based LIBs are currently investigated because the surface roughness and porous structure of paper and textile are ideal for ions transportation and the flexibility of the paper and textile benefits the realization of fully bendable batteries. However, conventional paper and textile substrates are not electrically conductive for electrons. Therefore, highly conductive paper for flexible batteries is strongly needed. Cui et al. demonstrated that conformal coating of single-walled carbon nanotubes (SWNTs) on the surface of paper and textiles can instantly turn the paper or textile into a highly conductive medium for electron transport.[22,23] Similar conductive fabric or paper has been demonstrated by others as well.[24–27] The realization of flexible conductive paper

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or textile will benefit the investigation of paper-thin, flexible, and lightweight LIBs. Carbon nanotubes (CNTs) have been widely used as electrodes in electrochemical devices due to their extreme flexibility and high conductivity.[23,28,29] Flexible batteries based on CNTs and their composites have also been developed in the past years and lots of significant results have been obtained. By combining CNTs with cellulose and an inexpensive insulating separator structure, Ajayan et al. fabricated porous cellulose paper embedded with aligned CNTs composite paper electrode. The composite paper exhibited superior flexibility and could be rolled up, twisted, even bent to any degree and was completely recoverable, which could be directly used as the flexible electrode for paper battery.[11] Using the nanocomposite paper as cathode and a thin evaporated Li-metal layer as anode, the assembled flexible paper battery exhibited a reversible capacity of 110 mAh/g and could be repeated over several tens of cycles of charging and discharging. However, in this work, Li metal was used as one electrode, which is not flexible and easily damaged during the bending process. Flexible paper batteries with conductive paper as the current collectors for both anode and cathode were later demonstrated.[15] During the fabrication process, no metal is used, which greatly benefits the realization of full flexible paper battery. As shown in Figure 2a, highly conductive SWNT films

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REVIEW Figure 2. Schematic illustration of: a) lamination process and b) the final paper Li-ion battery device structure. c) Picture of the Li-ion battery before encapsulation. d) Flexible Li-ion paper battery lightens an LED device. e,f) Galvanostatic charge–discharge curves of the laminated LTO-LCO paper battery (e), and self-discharge behavior of a full cell after being charged to 2.6 V (f). Reproduced with permission.[15] Copyright 2010, American Chemical Society.

were used as current collectors, and the active electrode materials were coated on the surface of the SWNT films. Figure 2b and 2c show the schematic illustration and the final paper battery device before encapsulation and cell testing. It is the first report of the use of commercial paper in LIBs, where paper is used as both separator and mechanical support.[15] Figure 2d shows a highly flexible, rechargeable paper battery lighting up an LED device. The first-cycle voltage profile of the paper battery is illustrated in Figure 2e. Using the CNT/ LTO and CNT/LCO as the anode and cathode, respectively, the full battery exhibits a discharge platform of 2.0–2.2 V. The cycle performance of the battery is shown in the insert of Figure 2f. The coulombic efficiency is 94–97% after the first cycle, and the discharge capacity remaining is 93% after 20 cycles. Moreover, the paper battery exhibits an excellent self-discharge performance with only a 5.4 mV voltage drop after 350 h. The superior electrochemical performance of the flexible paper battery renders it as candidate for flexible electronic devices.

2.2. Flexible Lithium-Ion Batteries Based on 3D Electrodes Commonly, the capacity and rate performance of an LIB depend critically on the active surface of the electrodes, the ions mobility in the electrolyte and diffusion in the electrode, and electrons transfer in the active electrode materials. Strategies to increase the surface area and the ion/electron transport kinetics in LIBs have mainly paid on searching new electrode materials and fabricating novel electrode structures with high active surface and high conductivity.[30–33] Among these strategies, fabricating electrodes based on three-dimensional (3D) architectures, that have large surface area, better permeability, and reduced path length, is regarded as a promising approach to obtain LIBs with high capacity and high rate capability.[29]

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To fabricate 3D flexible electrodes, flexible current collectors with 3D structures are also essential. Carbon cloth or textile, a soft collector with 3D network architecture that shows high electron conductivity, and robust mechanical stability, has been recently used as a replacement of conventional metal current collector in LIBs.[34–38] By using a simple hydrothermal method, Liu et.al. demonstrated a 3D ZnCo2O4 nanowire arrays/carbon cloth hierarchical structure as an integrated electrode for flexible LIBs.[19] The obtained ZnCo2O4/carbon cloth composite with highly ordered 3D ZnCo2O4 arrays could be grown on individual carbon microfiber (Figure 3a). Using the composite as binder-free anode and LiCoO2/Al foil as cathode, flexible full battery with operating window between 2.2 and 3.7 V was assembled. The as-fabricated flexible battery exhibited high capacity of about 1300 mA h g−1 and kept about 96% of its initial value after 40 cycles (Figure 3b). The packaged full battery can easily light an LCD mobile display even under bending state. The folding endurance is an important parameter to reflect the flexibility. No performance degradation was observed from the voltage profiles by bending the flexible battery for hundreds cycles (Figure 3c), revealing its excellent electrical and mechanical stability. Liu and co-workers also fabricated self-assembled ZnCo2O4 urchins on carbon fibers as the binder-free anodes for flexible LIBs, which showed a reversible capacity value of 1172 mA h g−1 in the voltage window 2.2–3.7 V at 0.2 C after 50 cycles.[35] Other flexible batteries with similar structure were also demonstrated based on different anodes, such as stannic oxide, metal germinate, and silicon nanowires;[36–38] however, the mass of carbon cloth is much heavier than metal foils such as copper and aluminum with the same area, which may decrease the mass specific energy density of the full battery to some extent. Thus light-weight and flexible electrodes or current collectors with 3D architectures are still a challenge. Very recently, Wang et al. fabricated a 3D web-like Li4Ti5O12 architecture on an ultrathin titanium foil (Figure 3d),

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Li4Ti5O12/GF electrodes with high rate performance due to the contribution of GF. The hybrid materials exhibited outstanding flexibility and could be bent into arbitrary shapes without degradation. Excitingly, the 3D interconnected network structure can also been completely preserved (Figure 4a,b). Figure 4c shows the Li4Ti5O12 nanosheets in the hybrid materials; these materials exhibit a long, flat potential plateau at 1.0 V and a high capacity of 86 mA h g−1 at 200 C. Using the flexible LiFePO4/GF and Li4Ti5O12/GF as electrodes, a thin, light-weight and flexible full LIB with excellent electrochemical performance is demonstrated. The flexible device can power a red-light-emitting diode (LED) even when bent revealing its high bendability (Figure 4d). The assembled battery shows a stable operating voltage of 1.9 V even under bending state (Figure 4e). The bendability is further investigated by comparing the performances under bent and flat states. After 20 bends to a radius of 5 mm, the capacity of the as-assembled flexible battery decreased less than 1% demonstrating the high flexibility and mechanical stability (Figure 4f). More importantly, by replacing all of the inactive components with lightweight GF, the full battery shows a high energy density of about 110 W h kg−1, exhibiting potential applications in light-weight, flexible, and high-rate LIBs. Figure 3. a) SEM images of the ZnCo2O4 nanowire arrays on carbon cloth. b) Charge–discharge curves of the flexible full battery. c) Flexibility of the full battery. a–c) Reproduced with permission.[19] Copyright 2012, American Chemical Society. d) SEM image of the obtained Li4Ti5O12 architecture. e) Charge–discharge curves at 2 C. f) Lighting an LED by the flexible full battery. d,e,f) Reproduced with permission.[39] Copyright 2014, Springer.

which was used directly as the binder-free anode for LIBs.[39] The as-fabricated LIB showed ultra-long cycling performance with capacities of 153 and 115 mA h g−1 at 2 C and 20 C after 5000 discharge–discharge cycles. Using the LiMn2O4 nanorods on stainless steel cloth as cathode, flexible full battery was successfully assembled with an discharge platform of about 2.4 V (Figure 3e). The device exhibited high rate capacity of 120 mA h g−1 at a rate of 20 C and could be operated well under different bending curvatures revealing excellent flexibility (Figure 3f). However, the use of metal substrates may also go against the realization of fully flexible batteries. Recently, a unique 3D graphene macroscopic structure: graphene foam (GF) was extensively studied in flexible energystorage devices due to the high conductivity, light weight, high specific surface area, and excellent flexibility.[40–44] Moreover, the electrical conductivity and macroscopic structure render the GF possesses a highly conductive pathway for electrons, a short ion diffusion length, and a fast transport channel for ion flux, benefiting the fast charge and discharge capability of LIBs. By composting the GF with LiFePO4 and Li4Ti5O12, Cheng et al. developed thin, light-weight, and flexible LiFePO4/GF and 4766

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2.3. Flexible Lithium-Ion Batteries Based on Solid Electrolytes

To fabricate the flexible batteries, liquid-type electrolytes have been mainly used. However, the use of liquidtype electrolytes may limit the realization of a really flexible LIB because their thermal stability, mechanical stability and cell safety should be carefully considered in a flexible battery based on liquid-type electrolytes.[20] To realize the full-fledged flexible batteries with high performance and robust safety, designing and synthesizing readily-deformable solid electrolytes are strongly desired to secure the safety of flexible cells.[45–48] In the past several years, plastic crystal electrolytes (PCEs), composed of lithium salts and plastic crystals, have been studied, and satisfying thermal stability and acceptable ion mobility have been obtained.[20] Meanwhile, PCE/polymer matrix-based gel polymer electrolyte (PCPE), a modified composite PCE, has also been extensively investigated and meaningful results have been obtained.[49,50] However, most of the as-studied PCEs are extremely plastic and show liquid-like behavior at room temperature, while the PCPEs still suffer from insufficient mechanical stability and are too thick, hindering their applications in flexible batteries with high safety. To achieve the safety and mechanical stability of flexible LIBs, Lee et al. developed a new class of highly thin, deformable, and

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cathode, Li2Ti5O12 as anode, and N-PEPC as both the electrolyte and separator membrane, flexible battery maintained high safety even under serious shape deformations. As shown in Figure 5c, the as-fabricated flexible battery can still work well even under severely wrinkled condition. The assembled battery exhibits very stable charge/discharge behavior with cycling up to 30 cycles under wrinkled state (Figure 5d). These results indicate that the N-PCPE is an advanced solid electrolyte for flexible LIBs with high safety and outstanding flexibility. The thickness of the as-fabricated N-PCPE is about 25 µm, much thinner than that of conventional PCPE film (more than 100 µm) Figure 4. a) Photograph of a free-standing flexible LTO/GF bending bent. b,c) SEM images of and X-PCPE film (about 210 µm). However, the LTO/GF. d) Photograph of a bent battery lighting a red LED device. e) Galvanostatic charge– it still cannot meet the demands for lightdischarge curves of the battery. f) Cyclic performance of the battery under flat and bent states. weight and thin-film shape of flexible LIBs Reproduced with permission.[18] Copyright 2012, National Academy of Sciences. required for nano/microelectromechnical systems at the nanoscale or microscale. Lithium phosphorus oxynitride electrolyte (LiPON), physically safety-reinforced N-PCPEs solid electrolyte.[20] In their work, a deposited as amorphous thin film electrolytes, has been recently polyethylene terephthalate (PET) nanowoven is used to improve explored as electrolyte in all-solid-state microbatteries.[51–53] the mechanical properties of the N-PCPE. Through the in situ UV-crosslinking of ETPTA monomer under the co-presence of Using the physically deposited LiPON as both a solid electrolyte PCE, the 3D polymer electrolyte matrix can be formed directly and separator, a thin-film flexible LIB based on all-solid-state inside the PET nanowoven skeleton. From Figure 5a, one can materials has been demonstrated.[53] The flexible LIB shows found that the PET porous nanowoven is well-impregnated with high robustness by turning on a blue LED under the bending the plastic crystal polymer electrolyte matrix. The ionic conducstate (Figure 6a). The cross-sectional structure of the solid-state tivity of the N-PCPE shows no significant decrease compared LIB on a mica substrate can be seen from Figure 6b, where with the EC/DMC-based liquid electrolyte at high temperature we can find that the thickness of the LiPON electrolyte is only of 80 °C (Figure 5b) and high deformability. Using LiCoO2 as about 2 µm, so thin that it can be widely used in all-solid-state, ultrathin, and flexible microbatteries. At various bending radius values, the performance of the battery did not show any external damage. With increasing the bending curvature, the specific capacity of 106 µA h cm−2 for non-bending state is gradually decreased to 99 µA h cm−2 at Rc = 3.1 mm, and the capacity loss is negligible during 100 charge and discharge cycles (Figure 6c). The slight reduction in charge–discharge capacities can be ascribed to the residual stresses caused by the bending as well as the volume change in the cathode resulted during the cycling processes. No voltage change of the battery at bending condition reveals the mechanical stability of the flexible thin-shaped battery (Figure 6d). Importantly, the bendable LIB can be integrated with a thin LED display to realize a bendable self-powered device, which provides an pregnant pathway for the emerging flexible integrated electronic systems. However, the solid state electrolytes suffer from low ionic conductivity, especially at low temperatures, which needs to be furFigure 5. a,b) Cross-sectional SEM image (a) and thermal stability of the N-PCPE (b). c) Photograph showing active state of a red LED lamp connected to the wrinkled cell. d) Charge–dis- ther resolved through exploiting new solidcharge profiles of the wrinkled cell with cycling (charge/discharge current densities = 0.2 C/0.2 state electrolyte materials with higher ionic conductivity at room/low temperature. C). Reproduced with permission.[20] Copyright 2013, Wiley.

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the first charge/discharge cycle (Figure 7b). Mechanical bending test (Figure 7c) reveals that the voltage of the battery could be stably maintained when the battery is bent to different states. A photograph of the cable LIB with excellent mechanical flexibility is shown in Figure 7d. The wire shape and omni-directional flexibility of the cable-type battery render this structural design free from conventional constraints. Moreover, compared with the conventional batteries mounted inside electronic devices, the cable battery can be adapted to fit nearly anywhere, thereby facilitating the realization of practical wearable electronics. Along with the fast development of flexible electronics, stretchable electronics are springing up as a new technological advancement due to their reversible stretchability while still maintain their functionality. Stretchability represents a rigorous challenging of mechanical stability. The Figure 6. a) Photograph of an LED powered by a flexible LIB under bending state. b) Cross- stretchable devices must bear large strain section of the LIBs on a mica substrate. c) Capacity retention of the flexible LIBs after 100 deformation, and large shape deformation, cycles under different states. d) Voltage retention during one bending cycle. Reproduced with including not only bending, but also twisting, permission.[53] Copyright 2012, American Chemical Society. stretching, compressing and others. Therefore, stretchable energy-storage devices are strongly desired to power the stretchable electronics.[59] In 2.4. Structural Design of Flexible Lithium-Ion Battery 2013, Rogers et al. introduced a high stretchable LIB with the In portable electronics, the shape of the battery is a particular active materials segmented design, and unusual ‘self-similar’ limiting factor for the creation of practical and aesthetic interconnect structures between them.[60] The stretchable LIB devices. Indeed, a groundbreaking technology to design novel was fabricated on a thin silicone elastomers as substrates, batteries with structure innovation can be achieved if the restriction of battery shape was broken.[54] Flexible batteries mentioned above are considered as promising solution to fabricate flexible batteries with alterative shapes. However, the large voltage, the structural limitations of the planar architecture, and the lack of desired omi-directional flexibility may limit their potential applications in electronic devices with special demands in the future.[55] To this end, wire-shaped LIBs reveal unique and promising advantages over conventional sheet-like batteries.[56] As an original concept for a battery architecture, cable-type LIBs might indeed provide the breakthrough necessary in flexible electronics because of their excellent mechanical flexibility. Recently, LG Chem. Ltd demonstrated a cable-type LIB[57,58] characterized by a hollow spiral, spring-like anode (comprising nickel/tin-coated copper wires) and a modified polyethylene terephthalate (PET) nanowoven separator membrane, and the cross-sectional structure of the cable battery is shown in Figure 7a. The cable battery Figure 7. a,b) Side view (a) and first charge–discharge–profiles (b) of the cable battery. c) Disexhibited a potential plateau at about 3.5 V charge characteristics with variations in bending strain. d) Photograph of the prototype cable and a specific capacity of 1.0 mA h cm−1 for battery used to power a red LED screen. Reproduced with permission.[57] Copyright 2012, Wiley.

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make them widely used in electronics, memory back-up systems, electric vehicles and hybrid electric vehicles and industrial power and energy management.[68–70] Recently, portable electronic devices, such as mobile phones, wearable electronics, and flexible displays with lightweight and flexibility are becoming favorite of the modern market. In this regard, flexible SCs have received increasing attention as a new kind of energy-storage devices due to the high specific/volumetric energy and power densities.[71–75] Compared with conventional SCs, flexible SCs show outstanding advantages such as smallsize, high-flexibility, light-weight, ease of handing, and wider range of operation temperature. Therefore, flexible SCs hold great promises for flexible and wearable electronics. Up to now, considerable work has been devoted to the fabrication of flexible SCs, and great advances have been achieved.[67,76–84] In this section, we review the recent progress on the development of flexible SCs device and also discuss the future challenges and opportunities for the fabrication of the state-of-the-art flexible SCs.

3.1. Flexible SCs Based on Pure Carbon Materials

Figure 8. a,b) Schematic illustration (a) and exploded view layout (b) of a completed battery. c) Galvanostatic charging and discharging of the battery electrodes without and with 300% uniaxial strain. d) Output power as a function of applied biaxial strain. e,f) Lighting a red LED: without strain (e) while biaxially stretched to 300% (f). Reproduced with permission.[60] Copyright 2013, Macmillan Publishers Limited.

as illustrated in Figure 8a,b. As a result, the as-fabricated LIB obtains a stretchability up to 300%, and still maintains a capacity of 1.1 mA h cm−2 at a rate of C/2 (Figure 8c). The output power of the battery decreased slightly with strain resulted from the increased internal resistances with strains at these large levels, as shown in Figure 8d. Surprisingly, the stretchable battery could turn on a commercial LED even when it was stretched by up to 300%. Figure 8e and 8f show the high stretchability of the battery which satisfies the requirements for many applications that are being contemplated for stretchable electronics.

3. Flexible Supercapacitors Supercapacitors (SCs), also known as electrochemical capacitors or ultracapacitors, have attracted considerable interest in recent years because of their relatively high energy density, large power density, and long cycling life (>100 000 cycles).[61–67] They are bridging the gap between conventional capacitors and batteries. In comparison to conventional capacitors, SCs show energy density several orders of magnitude higher. Furthermore, SCs can also store and deliver a large amount of charge in a short period of time, which allows them to provide higher power than batteries. These features

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Over the past few decades, pure carbon materials bring substantial opportunities for developing advanced flexible electrodes given that their high conductivity, high surface area (up to 2630 m2 g−1), lightweight, high temperature stability, controllable porous structure, compatibility in composite materials, and relatively low cost.[71,85,86] Various carbon materials such as active carbon, carbon particles (CNPs), carbon nanotubes (CNTs),[87–92] graphene[77,93–102] and their composites[103,104] have been extensively studied due to their unique physical and chemical properties. This section summarizes the recent developments of flexible SCs based on pure carbon materials.

3.1.1. Flexible CNT-Based SCs Compared with other carbon materials, CNTs are excellent electrode materials, especially for flexible SCs, due to its high specific surface area (1240–2200 m2 g−1), high electrical conductivity (104–105 S cm−1), and controllable regular pore structure.[89,105–108] Moreover, CNTs possess a high aspect ratio, which not only provides long continuous conductive paths, but also ensures high flexibility.[89] Kaempgen et al. reported printable thin-film SCs based on the single walled CNT (SWCNT)network-coated PET electrodes.[88] The SWCNT networks can offer not only a superior robustness in terms of bending and abrasion but also an enhanced conductivity (ca. 40–50 Ω/sq), which can be ascribed to the high fault tolerance since many different current pathways remain possible even with a few disconnected or missing links within the network. Consequently, the as assembled flexible SC device exhibited an energy density of 6 W h kg−1 in H3PO4/Polyvinyl alcohol (PVA) gel electrolytes. However, the smooth flat substrates only allowed a lower mass loading of CNT (0.03 mg cm−2) and the PET substrates showed poor bonding with the CNT, and film delaminating was observed. To increase CNT mass loading, Hu et al. prepared flexible paper-based electrodes by coating conductive SWCNT

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suspension on both sides of a piece of printing paper pre-treated by polyvinylidene fluoride (PVDF), and the CNT mass could achieve 0.3 mg cm−2.[15] Additionally, the SC device based on this kind of SWCNT/paper electrode has excellent flexibility. Recently, bacterial nanocellulose (BNC) papers were also used as substrates to fabricate flexible CNT electrodes. Kang et al. reported a vacuum-filtering process to coat a CNT layer onto the BNC surface.[90] The similar 1D structure of both BNC paper and CNTs make them intertwined into 2D sheets, which lead to a seamless interface. As a result, the CNT-coated BNC papers showed high mechanical stability over hundreds of bending cycles without being separated into individual layers. Moreover, the flexible allsolid-state SCs based on the-prepared BNC/ CNTs electrode and triblock-copolymer gel electrolyte were able to operate in a voltage of 3 V and have high energy and power density of 15.5 W h kg−1 and 1500 W kg−1 (measured at 1 A g−1), respectively. A similar flexible SCs with a voltage of 3 V was Figure 9. a) CV curves of the stretchable SC based on buckled SWCNT films under 120% also developed by Kang et al.[89] They coated strain at different scan rates. b) The specific capacitances of a stretchable SC under different [110] CNTs on office papers as flexible electrodes strains. a,b) Reproduced with permission. Copyright 2012, Wiley. c,d) SEM image of the 3D graphene hydrogel (c), and CV curves of the flexible solid-state device at 10 mV/s for different and used an ionic-liquid-based gel as elecbending angles (d). c,d) Reproduced with permission.[44] Copyright 2013, American Chemical trolyte to assemble SCs. The as-fabricated Society. solid-state SCs exhibited excellent electrochemical properties, stability and flexibility, with the maximum energy density of 41 W h kg−1 and power by integrating the robust, conductive, and free-standing Nafionfunctionalized reduced graphene oxide (f-RGO) electrodes and density of 164 kW kg−1. Zheng and co-workers prepared a kind solvent-cast Nafion electrolyte membranes.[115] The f-RGOof SSC device based on the TiO2@C core–shell nanowires on carbon cloth and PVA/H2SO4 gel electrolyte, which exhibits an based SCs showed a higher specific capacitance (118.5 F g−1 excellent flexibility that can even be folded and twisted without at 1 A g−1), anout two times of the pristine RGO-based SSCs destroying their electrochemical properties, and a maximum (62.3 F g−1 at 1 A g−1), showing the Nafion-functionalization energy density of 0.011 mW h cm−3 was obtained.[109] Recently, can significantly improve the electrochemical performance of the graphene. However, its capacitive properties of graphene Niu et al. synthesized highly stretchable buckled SWCNT films such as capacitance, energy density, and power density have by combining directly grown SWCNT films with polydimethylremained lower than expected,[97,116] which can be ascribed to siloxane (PDMS), and they reported their implementation as electrodes in flexible SCs.[110] The SC device based on the the restacking of graphene sheets during its processing due to the strong sheet-to-sheet van der Waals interactions.[62] buckled SWCNT films possessed excellent flexibility, and exhibited remarkably stretchable, whose performance showed littile Up to now, considerable effort has been dedicated to degradation when stretched under 120% strain (Figure 9a,b). the inhibition of restack of graphene, especially for energy storage.[62,82,87] For example, Yang et al. reported an effective bioinspired approach to prevent the restacking in multilayered graphene films and the use of these unrestacked graphene 3.1.2. Flexible Graphene-Based SCs paper as high-performance electrodes for flexible SCs.[99] These Graphene, which consists of single or few stacked ordered sp2 face-to-face-stacked multilayered graphene sheets possessed a highly open pore structure with large specific surface area, carbon sheets, have attracted substantial interest in flexible which allows the electrolyte solution easily access to the surSCs owing to its high electrical conductivity, and specific surface of individual sheets. As a result, flexible SCs based on face area (up to 2630 m2 g−1).[62,99,111–113] Yoo et al. developed these self-stacked, solvated graphene paper electrode exhibited ultrathin flexible SCs based on the composites of pristine a high specific capacitance of 273.1 F g−1 and a substantially graphene and multilayer reduced graphene oxide (RGO) and [ 114 ] PVA/H3PO4 gel electrolyte. high energy density up to 150.9 W h kg−1. El-Kady et al. recently These flexible SCs made from 1–2 graphene layers achieved a specific capacitance of up to developed a standard Light Scribe DVD optical drive to do the 80 µF cm−2 and could further improved after using more gradirect laser reduction of graphite oxide (LSG) films to graphene and demonstrated their applications as flexible electrodes in phene layers. Choi et al. also fabricated a flexible thin SC device

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3.1.3. Flexible-CNT/Graphene-Composite-Based SCs Recent studies demonstrate that hybrid materials of the 2D graphene sheets and 1D CNTs can exhibit synergistic effects such as greatly improved electrical, thermal conductivity, and mechanical flexibility compared with each single constituent component.[103,104] Very recently, Cheng et al.[117] synthesized 1D CNTs on 2D graphene (CNTs/G fiber). The flexible textile of CNT/G fibers was used as electrodes for construction of flexible SCs. Due to the high surface area, higher mechanical flexibility and electrical conductivity of CNT/G fiber electrode, the as assembled SCs displayed an outstanding capacitive performance at different bending states and bending cycles.

3.2. Flexible SCs Based on Composite Materials Carbon materials are known to store energy according to the electric double-layer effect at the electrode/electrolyte interface. Flexible SCs based on pure carbon materials generally deliver very high power density but relatively low energy density that limited by the low capacitance of carbon materials arising from the limited charge accumulation in electrical double layer. In

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order to increase the energy density of flexible SCs, a great deal of effort has been devoted to improving the capacitance of the electrodes. Pseudocapacitive materials that store energy through surface redox reactions exhibit substantially higher specific capacitances of 300–1200 F g−1 than carbon materials.[118–120] Composited electrodes made up of highly conductive materials and pseudocapacitive materials have received great interest for their superior electrochemical performances. Flexible SCs based on these composited electrodes can achieved both high energy and high power density.[119,121–125] In this section, we focus on the most recent work regarding the latest development of flexible SCs based on the composited electrodes. To push the energy-density limit of flexible SCs, a great deal of effort has been devoted to improving the capacitance by fabricating composited electrodes. In particularly, composites consist of carbon materials and pseudocapacitive materials such as MnO2,[103,106,118,122,126] RuO2,[127] polyaniline (PANI)[121,128–130] are the most promising and studied electrode materials for flexible SCs. This kind of composite can effectively take advantage of both the high conductivity of the carbon materials and the high specific capacitance of the pseudocapacitive materials, and hence result in significant improvements in both the energy density and the power density. Among metal oxides, MnO2 has been widely recognized as one of the most attractive electrode materials for SCs in terms of its high theoretical specific capacitance (ca. 1400 F g−1), low cost, natural abundance, and environmental compatibility.[131] In recent years, the fabrication of flexible SCs based on MnO2 electrodes have become a hot research spot.[131,132] For instance, a flexible solid-state SC device with an energy utilization efficiency of about 80% has been reported by using MnO2 nanorods grown on carbon cloth as electrodes.[132] However, the energy density of the flexible SCs based on pristine MnO2 electrodes is limited by its intrinsically poor conductivity (ca. 10−5–10−6 S cm−1). To improve the energy density and power density of flexible MnO2-based SCs, hybrid MnO2/carbon composites such as carbon nanoparticles (CNPs)/MnO2,[122] CNTs/ MnO2,[106,118] graphene/MnO2[126,133] and graphene/MnO2/ CNTs[103] have been actively explored. MnO2 was usually coated onto the surface of carbon materials via various methods. For example, Yuan et al. electrodeposited MnO2 onto the CNPs surface and demonstrated a high-performance flexible solid-state SC device with these CNPs/MnO2 electrodes and a PVA/H3PO4 gel electrolyte.[122] The device has good electrochemical performances with an energy density of 4.8 W h kg−1 (at 14 kW kg−1) and more than 97% retention of its initial capacitance after 10 000 cycles. Hu et al. reported a flexible aqueous SC device made from MnO2-CNTs-cotton electrodes achieved high areal capacitance of 0.48 F cm−2 and high specific energy density of 20 W h kg−1.[118] He et al. developed a freestanding 3D graphene/ MnO2 composite networks as electrodes for flexible aqueous SCs.[133] The freestanding, lightweight 3D graphene networks was prepared from the pressed Ni foam, exhibiting superior mechanical strength and flexibility. A composite 3D electrode could be obtained by coating a large and uniform mass of MnO2 onto the entire skeleton by using electrodeposition. The as-assembled flexible SC is demonstrated in Figure 10a. The device based on the 3D graphene/MnO2 composite electrodes showed high specific capacitance, excellent rate capability, and

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flexible SCs.[62] The LSG has an extremely large specific surface area of 1520 m2 g−1 and high conductivity of 1738 S m−1. Flexible SCs based on these LSG electrodes and an ionic liquid electrolyte delivered a maximum energy density of 1.36 mW h cm−3 and power density of up to 20 W cm−3. Niu and co-workers[93] prepared 3D reduced graphene (rGO) foams as high-performance electrodes in flexible SCs. Electrically conductive, lightweight and paper-like rGO foams obtained by autoclaved leavening and steaming of GO layered films achieved a specific capacitance of 110 F g−1and excellent flexibility. The synthesis of graphene hydrogel is another key method for designing 3D structure. Xu et al.[44] recently fabricated flexible solid-state SCs based on 3D graphene hydrogel films, as shown in Figure 9c. With a highly interconnected 3D network structure, the assynthesized graphene hydrogel exhibited exceptional electrical conductivity (192 S m−1), specific surface area (ca. 414 m2 g−1) and mechanical robustness. All these features make it very promising for flexible SCs. The as-assembled solid-state SC device using these 3D graphene hydrogel yielded outstanding capacitive performance in H2SO4/PVA gel electrolyte, such as a high specific capacitance of ca. 186 F g−1 (1 A g−1), good stability, high energy density of about 6.5 W h kg−1 and excellent flexibility (Figure 9d). They also fabricated flexible SC device by using the functionalized 3D graphene hydrogels, which exhibited enhanced specific capacitance of 441 F g−1 compared with the unfunctionalized graphene hydrogels (211 F g−1) at current density of 1 A g−1.[42] Additionally, the use of solidstate ionic liquid electrolyte can also improve the energy density of graphene-based SCs. Tamilarasan et al.[96] reported a mechanically stable, flexible graphene-based all-solid-state SCs with ionic liquid incorporated polyacrylonitrile (PAN/[BMIM] [TFSI]) electrolyte. By expanding the potential window to 3 V, the as-fabricated flexible SCs achieved a high energy density of 32.3 W h kg−1.

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on the whole device, Figure 10d), respectively. A similar flexible SC based on PANI/ single-walled carbon nanotubes (SWCNTs) composite electrode was designed by Wang et al.[128] and exhibited a high energy density of 26.6 W h kg−1 (based on electrode). To meet the development of transparent and flexible optoelectronic devices, Ge and co-workers used a kind of PANI/SWCNTs composite electrode to develop a transparent and flexible solid-state SC.[129] This transparent SC showed a specific capacitance of 55.0 F g−1 at a current density of 2.6 A g−1. Recently, Lin et al. also developed a kind of transparent and flexible SC based on PANI/ multiwalled carbon nanotubes (MWCNTs) composite films.[130] The PANI/MWCNTs films were synthesized via a simple electrodeposition process and achieved a maximum specific capacitance of 233 F g−1 at a current density of 1 A g−1. Metal nitrides such as titanium nitride [76] and vanadium nitride (VN)[125] have Figure 10. a) Schematic diagram and photoimages of flexible symmetrical SCs based on gra- (TiN) phene/MnO2 composite electrodes. b) CVs of the flexible supercapacitors at scan rates from received increasing attention as electrode 50 to 1000 mV s−1. a,b) Reproduced with permission.[133] Copyright 2013, American Chemical materials for flexible SCs. Lu et al. recently Society. c) SEM image of the PANI/CNT composite electrode. d) Ragone plots for the electrode reported a high-performance solid-state SC materials and for the entire all-solid-state device. c,d) Reproduced with permission.[121] Copydevice based on the stabilized TiN nanowire right 2010, American Chemical Society. electrode for the first time.[76] The fabricated TiN-based SC device achieved an excellent volumetric specific capacitance of 0.33 F cm−3 and a maximum long cycling performance. Moreover importantly, the fabricated flexible SCs based on these 3D graphene/MnO2 composite elecenergy density of 0.05 mW h cm−3. In addition, this device also trodes were able to operate in a voltage of 1 V and achieve an exhibited outstanding cycling performance that could retain energy density of 6.8 W h kg−1 at a power density of 62 W kg−1 83% of its initial capacitance after 15 000 cycles. In order to further enhance the energy density of metal nitrides, Xiao (Figure 10b). Similarly, Peng and co-workers also developed et al. synthesized a hybrid VN/CNTs composite electrode by a a flexible, high-performance, in-plane SC based on MnO2/ simple vacuum-filtering method.[125] The areal capacitance of graphene composites.[126] This planar solid-state SC delivered a high specific capacitance of 267 F g−1 at current density of this hybrid VN/CNTs electrode reached 178 mF cm−2 at current 0.2 A g−1 and excellent rate capability and cycling performance density of 1.1 mA cm−2. When using the hybrid VN/CNTs elecwith capacitance retention of 92% after 7000 cycles. Recently, a trodes to assemble a solid-state SC, the fabricated SC was able kind of flexible aqueous SC device based on interconnected grato operate in a voltage of 0.7 V and has good mechanical flexphene/MnO2/CNTs nanocomposite electrode has been demibility. Furthermore, this device achieved a high volume capacitance of 7.9 F cm−3 and energy density of 0.54 mW h cm−3. onstrated to possess ultrahigh performance by Cheng et al.[103] The synergistic effects from graphene, CNTs, and MnO2 enaIn addition to the composites of carbon materials and pseudocapacitive materials, other composites of pseudocapacitive bles efficient charge transport and electrode integrity, endowing materials with other conductive materials such as Au,[120,134] the films with outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that WOx,[123] ZnO,[124] TiO2,[135] CoAl-layered double hydroxide[119] were not achieved by any of these components alone. The SC and conducting polymer such as polypyrrole (PPy)[136] have device assembled from the graphene/MnO2/CNTs nanocombeen explored to fabricate flexible SCs. To improve the electric conductivity of MnO2, Lang et al. posite electrode delivered remarkable specific capacitance of 70 F g−1 at 10 mV s−1 and high energy density of 8.9 W h kg−1 used nanoporous Au as highly conductive material to support MnO2.[134] The nanoporous Au enables the fast electrons at 106 W kg−1. transport through the MnO2, and can greatly facilitate the ion Hybrid composites of carbon materials and conducting polymers have been also extensively studied in flexible diffusion between the MnO2 and the electrolytes, and thus SCs.[74,121,128–130] Meng et al. developed an ultrathin all-solidexhibiting an ultrahigh specific capacitance of 1145 F g−1. Addistate paper-like polymer supercapacitor with a total thickness of tionally, the energy and power densities of flexible SCs based 100 µm.[121] The device was assembled by solidifying two pieces on this kind of hybrid Au/MnO2 electrode increased with the of PANI/CNT composite films (Figure 10c) in a PVA/H2SO4 gel loading amount of MnO2, and achieved maxima of 57 W h kg−1 electrolyte. The maximum energy density and power density of and 16 kW kg−1, respectively. Recently, Lu et al. also demon−1 −1 this device achieved are 7.1 W h kg and 2189 W kg (based strated a kind of high-performance flexible SCs by using hybrid

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10 000 cycles, and high flexibility with little performance degradation even under bent and twisted states (Figure 11e). Conducting-polymer-based composites have also been investigated as high-performance electrode materials for flexible asymmetric SCs.[120] For example, an ultrathin all-solid-state flexible SC device with a thickness of less than 1 µm, based on the nanoporous Au/PPy composite was reported by Meng and co-workers.[120] Nanoporous Au/ PPy composites were obtained by a convenient dealloying and electropolymerization process. The overall thickness of the as-fabricated SCs based on these Au/PPy composites was only about 600 nm, which is typically one or two orders of magnitude thinner than other reported flexible SCs. Due to the fast responses to ions and electrons, this device was able to deliver an ultrahigh volumetric energy density of 2.8 mW h cm−3 and power density of 56.7 W cm−3. Furthermore, this device also exhibited extraordinary flexibility and cycling stability. Yu et al. recently reported the facile synthesis and improved electrochemical performance of the TiO2@ PPy core–shell nanowires.[135] Electrochemical measurements show that the TiO2@PPy core–shell nanowires on carbon cloth exhibFigure 11. a) Schematic of the fabrication process for WO3–x@Au@MnO2 NWs. b) SEM image ited a high areal capacitance of 64.6 mF cm−2 of the products. c) Optical photograph of theflexible SC. a–c) Reproduced with permission.[123] at a scan rate of 10 mV s−1. Moreover, the Copyright 2012, Wiley. d) SEM image of the H-ZnO@MnO2 nanowires. e) CV curves of the direct growth of nanowires on carbon cloth SC collected under different bent states at a scan rate of 100 mV s−1. d,e) Reproduced with render the integrated electrode a high surface [ 124 ] permission. Copyright 2013, American Chemical Society. area, and can be directly used as the electrode for flexible SC without any additives. The solid-state SC device based on the as-prepared TiO2@PPy WO3–x@Au nanowires as highly conductive core to support MnO2.[123] Amorphous MnO2 was electrodeposited onto the nanowire electrodes exhibited good flexibility and achieved a maximum energy density of 0.013 mW h cm−3. Very recently, surface of WO3–x@Au nanowires grown on carbon cloth, as demonstrated in Figure 11 a and b. These WO3–x@Au@MnO2 a kind of conductive PPy/paper has been successfully synthesized by a simple “soak and polymerization” and shows great core–shell nanowires achieved an ultrahigh specific capacitance potential as flexible electrode for solid-state SCs.[136] The fabriof 1195 F g−1 at current density of 0.75 A g−1 with outstanding long-term cycling performance. Moreover, these WO3–x@ cated PPy/paper composite has a high electrical conductivity of 15 S cm−1 and a low sheet resistance of 4.5 Ω sq−1. The average Au@MnO2 core–shell nanowires have a remarkable energy −1 −1 density of 106.4 W h kg at power density of 23.6 kW kg weight of the flexible solid-state SCs assembled with the PPy/ paper composite electrodes was only about 55 mg, and could and a high power density of 30.6 kW kg−1 at energy density of operate in a voltage of 0.8 V. This SC device was able to achieve 78.1 W h kg−1. Additionally, the solid-state SCs assembled by an areal capacitance of 0.42 F cm−2 and a high energy density two WO3–x@Au@MnO2 nanowire electrodes and PVA/H3PO4 gel electrolyte represent a high flexibility that can even endure of 1 mW h cm−3 at power density of 0.27 W cm−3. In addifolding and twisting without affecting their performances tion, they also developed a kind of flexible PANI-based SCs by (Figure 11c). Similarly, Yang et al. electrodeposited amorphous using Au/paper as a conductive substrate to support PANI.[137] MnO2 onto hydrogenated ZnO (denoted as H-ZnO) nanowires This PANI-based SCs was also able to operate in a voltage of 0.8 V and exhibited a higher volumetric energy density of grown on carbon cloth to obtain H-ZnO@MnO2 core–shell 10 mW h cm−3 at power density of around 3 W cm−3. nanowires (Figure 11d).[124] These H-ZnO@MnO2 nanowires showed excellent electrochemical performance with a high Layered double hydroxides (LDHs) are promising pseudoareal capacitance of 138.7 mF cm−2 (1260.9 F g−1). By using capacitive materials owing to their high redox activity, low cost and environmentally friendly nature. On the other hand, as a these H-ZnO@MnO2 nanowires as electrodes and PVA/LiCl derivative of polythiophene, poly(3,4-ethylenedioxythiophene) gel as electrolyte, the fabricated flexible solid-state SCs exhib(PEDOT) has received considerable attention as an electrode ited an areal capacitance of 26 mF cm−2, good cycling performaterial due to its large electroactive potential window and mance with 87.5% retention of the original capacitance after

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high chemical stability among conductive polymers.[119] Han et al. reported the use of CoAl-LDH nanoplatelets grown on flexible Ni foil substrate as core to support electrochemically active PEDOT and demonstrated their prominent performance in flexible SCs.[119] The LDH@PEDOT core–shell nanoplatelets showed a remarkable specific capacitance of 649 F g−1 at 2 mV s−1 and energy density of 39.4 W h kg−1 at current density of 40 A g−1. Additionally, the hybrid LDH@PEDOT electrode has superior rate performance and delivers outstanding longterm cycling stability (92.5% retention of its initial capacitance after 5000 cycles).

3.3. Flexible Asymmetric SCs Besides improving the capacitance of electrode, the energy density (E) of flexible SCs can also be increased by maximizing the operation voltage. For this purpose, flexible SCs based on organic electrolyte or ionic-liquid-electrolytes have attracted considerable attention since the organic electrolytes or ionicliquid-electrolytes can provide a wider voltage (ca. 2–3 V) than aqueous electrolytes.[62,75,84] However, they usually suffer from high cost, poor ionic conductivity and high toxicity, which could hinder their applications. A promising alternative to increase the cell voltage is to develop asymmetric supercapacitors (ASCs) using aqueous electrolytes that have higher ionic conductivities and are more environmentally friendly. In comparison to symmetric supercapacitors (SSCs), ASCs are able to be operated in much wider potential windows.[63,67,73,82,138–140] ASCs usually consist of a battery-type Faradic electrode as energy source and a capacitor-type electrode as power source, which have the advantages of both supercapacitors (rate, cycle life) and advanced batteries (energy density). Moreover, ASCs can effectively make use of the different potential windows of the two electrodes to increase the maximum operation voltage (up to 2V even in aqueous electrolyte), and thus significantly enhancing the device energy density. In this regard, flexible ASCs have received increasing interest in recent years.[67,73,77,82,138–142] In this section we will introduce the recent achievement on the development of flexible ASCs. Transition metal oxides and Carbon materials are commonly employed as cathodes and anodes in flexible ASCs due to their complementary working potential windows.[73,77,81,83,140,143] In particular, the combination of a MnO2 cathode and a carbon anode is the most attractive and used system in flexible ASCs due to the high theoretical specific capacitance, low cost, abundance and environmental friendly nature of MnO2.[73,82,140,144,145] To date, a variety of flexible ASCs based on the MnO2 cathode and carbon-based anode have been developed and this kind of flexible ASCs can be able to deliver a wide voltage window of ca. 1.5–2.0 V.[73,82,83,138–141,143,144,146] Yu et al. used MnO2/graphene-textile as the cathode and CNT-textile as the anode to fabricate an ASC device. This device can operate in a 1.5 V voltage window in 0.5 M Na2SO4 and achieve a maximum energy density of 12.5 W h kg−1 with excellent cycling stability.[144] Similarly, Sumboja et al. prepared a kind of MnO2/ reduced graphene oxide (RGO) paper electrode with large areal mass of MnO2, and assembled an ASC device based on the RGO/MnO2 paper as cathode, RGO paper as anode and 1 M 4774

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Na2SO4 as electrolyte.[82] This ASC device exhibited a maximum areal energy density of 35.1 µW h cm−2 at the areal power of 37.5 µW cm−2 when it is operated in a voltage of 1.5 V. This study contributes significant advancement in the fabrication of thicker and large mass flexible electrodes without sacrificing their electrochemical performance. Shao et al. fabricated an aqueous flexible ASC device with a voltage of 1.8 V by using a graphene/MnO2 nanorod thin film as the cathode and a graphene/Ag thin film as the anode.[140] The ASC device achieved a maximum energy density of 50.8 W h k g−1 at a power density of 101.5 W kg−1, and the maximum power density was 24.5 kW kg−1 at an energy density of 12.3 W h kg−1, respectively. Recently, flexible aqueous ASC devices with a higher voltage of up to 2 V have been reported.[141,143,147] These devices have proven to be able to deliver much higher energy density and power density. For instance, the graphene/ carbon-nanotubes/MnO2-based ASC device exhibited the maximum energy and power densities of 33.71 W h kg−1 and up to 22.7 kW kg−1, respectively.[137] The ASC device based on a MnO2-PEDOT:PSS cathode and an active carbon anode can deliver a maximum volumetric energy density and power density of 1.8 × 10−3 W h cm−3 and 0.38 W cm−3.[141] The achievement of gel polymer electrolytes enables the fabrication of flexible quasi-solid-state/solid-state ASCs. In recent years, a number of different kinds of flexible solid-state ASCs based on MnO2 cathodes have also been developed.[73,81,139,146] Lu et al. reported the first fabrication of high-performance flexible solid-state ASCs based on a hydrogenated TiO2 (denoted as H-TiO2)@MnO2 core–shell nanowire cathode and an H-TiO2@C core–shell nanowire anode.[73] H-TiO2 nanowires grown on carbon cloth served as the conductive core to support amorphous MnO2 and carbon shells and exhibited enhanced electrochemical performances. The stable operation voltage of as-fabricated solid-state ASC device using H-TiO2@MnO2 nanowires as cathode and H-TiO2@C nanowires as anode in a PVA/ LiCl gel electrolyte can be extended to 1.8 V (Figure 12a). The volumetric and specific capacitances of the solid-state ASC device are comparable to the aqueous device and achieved a remarkable volumetric and specific capacitance of 0.71 F cm−3 and 141.8 F g−1 at 10 mV s−2 (Figure 12b). The maximum volumetric energy density and power density of this ASC device reached 0.30 mW h cm−3 (59 W h kg−1) and 0.23 W cm−3 (45 kW kg−1). Moreover, this ASC device also has excellent cycling stability and outstanding mechanical flexibility that can be folded and twisted without losing its electrochemical performance. Gao et al. designed a kind of flexible solid-state ASCs based on free-standing carbon nanotube/graphene and Mn3O4 nanoparticles/graphene paper electrodes with a polymer gel electrolyte of potassium polyacrylate/KCl.[81] The optimized ASC devices were able to operate in a voltage of 1.8 V and afford a maximum energy density of 32.7 W h kg−1 at 0.5 A g−1 with good cycling performance. Recently, Wang et al. used the ZnO@MnO2 cathode, RGO anode and PVA/LiCl gel electrolyte to develop a flexible solid-state ASC device with a voltage window of 1.8 V.[146] This device can achieve a high volumetric capacitance of 0.52 F cm−3 and a maximum volumetric energy density of 0.234 mW h cm−3 at 0.5 mA cm−2. In addition to MnO2//carbon system, other systems such as RuO2//graphene,[77] PANI//WOx@MoOx,[63] VOx//VN[67] and

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et al.[64] These porous VN nanowires grown on carbon cloth (Figure 12c) exhibited an excellent specific capacitance of 298.5 F g−1 at the scan rate of 10 mV s−1. Their study revealed that the use of a neutral PVA/LiCl gel electrolyte can significantly improve the stability of VN nanowires without sacrificing their electrochemical performance. Electrochemical studies showed that the ASC device using these stabilized VN nanowire as anode and VOx nanowires as cathode (Figure 12c) was able to operate in a potential voltage window between 0 and 1.8 V (Figure 12d) with 87.5% retention of initial capacitance after 10000 cycles. A remarkable volumetric capacitance of 1.35 F cm−3 and specific capacitance of 60.1 F g−1 were achieved by this VOx//VN-ASC device at current density of 0.5 mA cm−2. Additionally, the VOx//VN-ASC device has a remarkable rate capability, which can retain more than 74% of the initial capacitance when the scan rate increased from 0.5 to 5 mA cm−2. Moreover, Figure 12. a) CV curves of the solid-state H-TiO2@MnO2//H-TiO2@C-ASC device collected in the ASC device achieved a maximum energy different scan voltage windows. b) Volumetric and specific capacitance of the device calculated density of 0.61 mW h cm−3 at 0.5 mA cm−2 from the CV curves as a function of scan rate. a,b) Reproduced with permission.[73] Copyright and a high power density of 0.85 W cm−3 at 2013, Wiley. c) Photographs of carbon cloth substrates coated with VOx and VN nanowires. −2 −1 d) CV curves collected for VN and VOx nanowire electrodes at a scan rate of 10 mV s . 5 mA cm . These values are substantially [ 64 ] higher than most of the reported quasi/allc,d) Reproduced with permission. Copyright 2013, American Chemical Society. solid-state SC devices. Recently, Xu et al. developed a new kind of flexible ASC with cobalt sulfides (Co9S8) as the anode and MnO2/PEDOT//PEDOT[148] have also been explored in constructing high-performance flexible ASCs. For example, Choi Co3O4/RuO2 nanocomposites as the cathode.[142] Co9S8 nanorod and co-workers developed a flexible all-solid-state ASC device arrays were prepared on a carbon cloth by a hydrothermal sulusing an ionic liquid functionalized chemically modified grafuration treatment of acicular Co3O4 nanorod arrays, and the phene (IL-CMG) film anode, a hydrous RuO2-ILCMG comRuO2 was directly deposited on the Co3O4 nanorod arrays. posite cathode and a PVA/H2SO4 gel electrolyte.[77] This optiGiven that the Co9S8 and Co3O4/RuO2 electrodes have stable mized ASC device was able to operate in a voltage of up to 1.8 V voltage windows between −0.3 and 0.6 V and between −1.0 and and delivered a high energy density of 19.7 W h kg−1. Moreover, 0 V (vs SCE), respectively. The stable electrochemical windows of the Co3O4/RuO2//Co9S8-ASC device in both 3 M KOH this ASC device processed outstanding rate capability that can be operated even under an extremely high rate of 10 A g−1 with aqueous electrolyte and PVA/KOH electrolyte can be extended to 1.6 V. Electrochemical results show that the volumetric capac79.4% retention of specific capacitance. itance of the optimized ASCs achieved as high as 3.42 F cm−3 To further improve the energy density limit of flexible ASCs, great effort has been devoted to exploring new anode with high in aqueous electrolyte and 4.28 F cm−3 in PVA/KOH electrolyte capacitance and high conductivity.[63,67,148] Molybdenum oxide at a current density of 2.5 mA cm−2 (Figure 13a). Additionally, (MoOx) has substantially higher capacitance than carbon-based the aqueous and solid-state ASC devices were able to deliver a maximum energy density of 1.21 mW h cm−3 at 13.29 W cm−3 materials but suffers from poor conductivity. To solve this problem, Xiao et al. reported the use of oxygen-deficient tungand 1.44 mW h cm−3 at 0.89 W cm−3, respectively. Furthersten oxide (WOx) nanowires as a scaffold to load electrochemimore, the as-fabricated solid-state Co3O4/RuO2//Co9S8-ASC cally active MoO3-x, and demonstrated their superior perfordevice possessed remarkable flexibility as the shape of cyclc voltammetry (CV) curves retained unchanged under normal, mance as anode in flexile solid-state ASCs.[63] The ASC device bent, and twisted conditions (Figure 13b). Using the Co3O4 with the as-synthesized WOx@MoOx nanowires as anode, PANI nanowires as cathode, and PVA/H3PO4 gel as the elecnanowires on nickel fiber (Figure 13c) with operating window between 0.0 and 0.5 V as the positive electrode and graphene trolyte could operate in a stable voltage of 1.9 V with exhibits a coated on carbon fibers with voltage window from −1.0 to high areal capacitance of 216 mF cm−2 and a maximum energy –0.3 V as the negative electrode, Wang et al. fabricated an alldensity of 1.9 W h cm−3 at the power density of 0.035 W cm−3. solid-state fiber-based flexible ASC device, which can be operOwing to its large specific capacitance (1340 F g−1), high elecated up to 1.5 V (Figure 13d).[149] The volumetric capacitance trical conductivity and suitable working window, vanadium nitride (VN) have been considered as a promise anode for ASCs. A new of the device increased gradually from 0.234 to 0.695 F cm−3 kind of solid-state ASC device with high energy density based with the potential increased from 0.6 to 1.5 V, thus the energy on porous VN nanowire anode was recently demonstrated by Lu density can be increased at least by 1860%. The volumetric

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of interdigital microelectrodes, because of the shortened path lengths for ion diffusion and more effective utilization of the electrochemical active surface of the electrode materials. Wang et al. designed a flexible microSC microelectrode based on the patterned PANI nanowires array fabricated by microfabrication technology and in situ chemical polymerization approach.[165] By using a photolithography technology, the interdigitallike microelectrode was fabricated on a flexible PET chip, followed with thermal evaporating Au/Cr layer on the pattern as current collector. PANI nanowires arrays were deposited in situ on the Au layers, and PVA/ H2SO4 gel electrolyte was drop-cast onto the active materials to form micro-SCs. The flexible micro-SC acquired super volumetric capacitance of 588 F cm−3 and good rate capability. Using similar fabrication technology, Si has also been developed for a flexible [162] Figure 13. a) Rate capability of the solid-state Co3O4/RuO2//Co9S8-ASC device at different cur- micro-SC based on MnOx/Au multilayers. rent densities. b) CV curves collected at the scan rate of 100 mV/s for the solid-state ASC device The as-fabricated micro-SC exhibited a under normal, bent, and twisted conditions. a,b) Reproduced with permission.[142] Copyright maximum energy density of 1.75 mW h cm−3 2013, American Chemical Society. c) SEM image of the Co3O4 nanowires on a nickel fiber. d) CV and a maximum power density of curves of the ASC assembled using Co3O4 nanowires and graphene with the increase of the 3.44 W cm−3, which are both much higher potential window. c,d) Reproduced with permission.[149] Copyright 2014, Wiley. than the values obtained for other solid-state SCs. Furthermore, CV curves measured without strain and in bending states showed a similar capacicapacitance based on the total volume of the device reached tive behavior, with a capacitance loss less than 0.09%, demon2.1 F cm−3 at 20 mA cm−3, and maximum volumetric energy strating its high stability. To increase the energy density and density of 0.62 mW h cm−3 and power density of 1.47 W cm−3 power density of the micro-SCs, rGO was selected and used as were also achieved. the active electrode. By combining photolithography with selective electrophoretic techniques, Niu et al. fabricated ultrathin rGO interdigitated microelectrodes on a PET substrate coated 3.4. Flexible Micro-SCs with Au film (Figure 14a), and micro-SCs were also achieved using PVA/H3PO4 as a gel electrolyte (Figure 14b).[167] The speThe recent rapid advance and emergency of miniaturized, portable consumer electronics stimulate the development of microcific capacitance of the rGO micro-SC was about 285 F g−1 at scale power sources with high power density, towards the trend a scan rate of 5 mV s−1, much higher than that of the of conof being small, thin, lightweight, flexible, and even wearable, ventional rGO SCs (Figure 14c). The capacitances normalized to meet the growing demands of modern society.[150–152] Conto the geometrical area and volume of the rGO micro-SC are 462 µF cm−2 and 359 F cm−3. The calculated volumetric energy ventional SCs, however, are too large for very small devices and conventional manufacturing methods are not compatible with density is 31.9 mW h cm−3 and the maximum volumetric [ 153 ] microelectronic fabrication. power density is 324 W cm−3. Significantly, the micro-SC on the As one type of newly developed miniaturized electrochemical energy-storage devices, microPET substrate can be operated under bending without obvious SCs can offer power densities that are much larger than those CV performance deviation (Figure 14d), demonstrating that the of conventional batteries and SCs due to the short ion diffusion rGO-based micro-SCs are quite stable under the bending state, length.[154–156] Recently, great effort has been paid to increase and are suitable for flexible device applications. the energy and power densities of micro-SCs by using the As one of the flexible SCs, great interest has also been paid nanomaterials as the electrode. Various nanostructured mateto stretchable SCs due to their application in stretchable sysrials, such as onion-like carbon,[154] carbide-derived carbons,[157] tems. Li et al. developed a stretchable SC by using SWCNTs coated on PDMS as the electrodes and electrospun polyuregraphene,[156,158,159] hydrated graphite oxide,[155] graphene thane as the elastomeric separator.[168] The specific capacitance quantum dots,[160] MnO2,[161–163] PPy,[164] and PANI,[165,166] have been utilized as electrode materials for micro-SCs. of the stretchable SC is 50 F g−1 at scan rate of 100 mV s−1. Important for flexible miniaturized energy-storage devices, Interestingly, when a strain of 31.5% was applied, the capaciflexible micro-SCs on to a chip can be integrated with microtance of the device improved compared with that of the original electronic flexible devices to work as stand-alone power sources unstretched state. However, as for the stretchable micro-SCs, or as efficient energy-storage units, to which great interest has it is difficult to fabricate interdigital microelectrodes using been paid recently. Commonly, the micro-SCs exist in the form micro–nano fabrication technologies on stretchable substrates.

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consumer-grade LightScribe DVD burner.[159] In their report, GO film was first coated on the disc, followed by converting into graphene selectively using the photothermal effect. Thus the patterned-graphene-interdigitated electrodes with nearly insulating GO film as the separator can be obtained by using the precision of a laser. Unlike conventional microfabrication methods, this direct ‘writing’ technique is very simple and does not require masks, expensive materials, and other complex post-processing. Furthermore, the technique is cost effective and readily scalable. This technique has the potential for the direct writing of micro-SCs with a high areal density (Figure 14e). These micro-SCs exhibit an ultrahigh power of 200 W cm−3 with high flexibility and can be bent and twisted without the structural integrity disruption. As shown in Figure 14f, CV curves of the micro-SC with different bent and twisted states at 1000 mV s−1 showed superior electrochemical stability even under bending and twisting conditions, indicating its excellent flexibility and mechanical stability. Different from the planner micro-SCs on one-chip, flexible fiber-shaped SCs (FSCs), other micro-SCs, showing incomparable advantages for direct use as flexible, wearable and embedded device units have sprung up recently. In the past several years, many materials with novel structures, such as pen inks,[170] CNT fibers,[171,172] graFigure 14. Optical images of: a) rGO patterns on PET with Au film and b) micro-SC on PET. [173,174] Co3O4 nanowires,[149] and other c,d) The specific capacitance (c) and CV curves (d) at different bending states of the rGO phene, [ 175–178 ] [ 167 ] have been designed to micro-SC. a–d) Reproduced with permission. Copyright 2013, Wiley. e) Photographs of the composites, direct writing of a microdevice. f) CVs collected under different bending and twisting conditions fabricate flexible FSCs, exhibiting excelat 1000 mV s−1. e,f) Reproduced with permission.[159] Copyright 2013, Macmillan Publishers lent electrochemical performance and high Limited. mechanical stability. In 2012, Zou’s group introduced a novel flexible FSC that consists of two fiber electrodes, a helical spacer wire, and an elecTo achieve mechanical stability on stretchable substrates, Kim trolyte (Figure 15a).[170] In their work, commercial pen ink is et al. developed a 2D planar SC array on an a PDMS substrate. They designed long and narrow serpentine interconnections in employed as the active material for the first time, and remains a neutral plane and a planar micro-SC array.[169] The 2D planar robust after 15 000 electrochemical cycles. Flexible FSC using Au-coated plastic fiber as the current collector and substrate SC was fabricated by using patterned SWCNTs as electrodes and a PVA/H2SO4 gel electrolyte as the separator demonstrated and an ionic-liquid-based triblock copolymer as the electrolyte. A capacitance of 100 µF was obtained at a scan rate of 0.5 V s−1, high flexibility (Figure 15b). The FSC capacitance dropped only slightly after the high bending, revealing its good mechanical and no obvious damage or defects were observed in all the parts performance. CNT fibers have been widely used as the active of the device, including the SWCNT electrodes and interconelectrodes for FSCs due to their high conductivity and excelnections after stretching by 30%, confirming their potential lent mechanical stability. To improve their electrochemical perapplication in various future flexible and stretchable electronics. formance, many composites based on CNT fibers have been For the mentioned micro-SCs, the fabrication methods comdesigned. Choi et al. deposited MnO2 particles on to CNT bunmonly involve the photolithography technologies or employ masks for the patterns on the substrates, which are awkward dles to form CNT/MnO2 composite yarn SCs.[175] The highest and expensive for building cost-effective devices with large area volumetric capacitance of about 25.4 F cm−3 was when PVA/ for practical applications. Therefore, developing simple and KOH was used as the gel electrolyte. Bending cycling tests low-cost technologies that don’t require masks or other comshowed that there was no significant capacitance drop after the plex operations while producing high-performance micro-SCs 1000th bending at a 90° bending angle (Figure 15c). In addiare strongly desired. El-Kady et al. introduced a direct fabrication, a flexible composite FCS based on a novel multiwalled tion of micro-SCs based on the interdigitated graphene using a MWCNT/ordered mesoporous carbon (OMC) composite fiber

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both energy generation and energy storage may provide a novel method of developing a new mobile power source for both selfpowered systems and portable and personal electronics.[182] Recently, self-charged LIBs have been achieved using a piezoelectric nanogenerator[182,183] and dye sensitized solar cells (DSSCs).[184] Figure 16a shows nanogenerator-powered LIBs. In this hybridized system, the mechanical energy is directly converted and simultaneously stored as chemical energy. Also, there has been much progress in self-charged SCs and energy-harvesting devices mainly focus on DSSCs. Xu et al.[185] presented a novel stack-integrated photo-supercapacitor (PSC) thin-film device composed of a DSSC and an SC built on bipolar anodic titanium oxide (ATO) nanotube arrays. The optimized PSC exhibits an Figure 15. a,b) Schematic diagram (a) and photograph (b) of a flexible FSC. a,b) Reproduced impressive overall photoelectric conversion with permission.[170] Copyright 2012, Wiley. c) CV plots of solid-state CMY SC before and after and storage efficiency up to 1.64% with a a bending test. c) Reproduced with permission.[176] Copyright 2013, Wiley. d) Microscopy image fast response. Moreover, no performance of an EDLC wire woven into a polyurethane textile. d) Reproduced with permission.[177] Copy- degradation during the 100 photocharge/ right 2013, Wiley. galvanostatic discharge cycles reveals its excellent cycling capability. Furthermore, integrated self-charged SCs containing fiber-shaped DSSCs is developed utilizing MWCNT fibers for rapid charge separaand fiber-based SCs have been studied extensively and exhibit tion and transport and high surface area of OMC for high specific capacitance.[176] The as-fabricated twisted FSC showed the highest capacitance of 39.67 mF cm−2 at an OMC weight percentage of 87%, and exhibited a high performance retention after bending 1000 times. Interestingly, the twisted FSC could be woven into textile structures (Figure 15d), which have potential applications in future wearable electronics.

4. Integrated Energy-Storage Systems Integrated systems have been developed extensively in recent years because of their diversified functions with respect to the conventional devices with a single function. Integrated energy-storage devices, as one member of the integrated system family, have drawn great attention due to their significant importance for specific applications including self-powering systems, microelectromechanical systems, and portable/ wearable personal electronics.[179–181] In this section, we will briefly introduce the recent achievements in integrated energy-storage systems. Generally, energy generation and energy storage are two distinct processes and usually worked as separate devicesA achieving selfcharging in integrated devices combining

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Figure 16. a) Schematic diagram showing the design and structure of the self-charging LIB. a) Reproduced with permission.[182] Copyright 2012, American Chemical Society. Schematic diagram of the fiber-based self-charged SC in the process of charging and discharging. b) Reproduced with permission.[188] Copyright 2014, Wiley. c) Schematic illustration of the DSSC-driven EC device. d) Images of the device in the colored and bleached states. c,d) Reproduced with permission.[189] Copyright 2014, The Royal Society of Chemistry.

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5. Conclusions and Future Outlook In this review, the recent progress in flexible LIBs and flexible SCs are summarized systematically. The fabrication of flexible energy-storage devices requires a flexible current collector, flexible electrode materials, a flexible solid-state electrolyte, and a flexible encapsulating material. Although great achievements regarding these factors have been obtained by the mentioned flexible energy-storage devices, many drawbacks still exist and need to be conquered to improve the performance of the fabricated flexible LIBs and SCs. For example, as for flexible LIBs, commonly used organic liquid-type electrolytes bring a number of safety problems because of their thermal stability, whereas solid state electrolytes suffer from low ionic conductivity. Moreover, the real sense of solid-state SCs needs to be achieved because the as-mentioned flexible SCs usually employ a gel electrolyte, which is in a quasi-solid state. To further improve the performance of flexible energy-storage devices, the following research aspects should be considered and more work should be launched to conquer the current drawbacks.

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i)

Although a series of flexible energy-storage devices have been fabricated, the energy and power densities still need to be improved. Discovering new electrode materials with a high specific surface area, a short ion-transfer path and pore network improving the active surface and the diffusion of ions, and designing novel, 3D and binder-free electrode structures with fast electron transfer more electrochemical active surface may be important research directions. ii) More attention should be paid to flexible energy-storage devices with novel structural designs to meet the different demands of flexible electric devices. A noteworthy technological advance that has recently received considerable attention is fiber-shaped LIBs and SCs with extreme omni-directional flexibility, which frees the cell designer from rigid constraints.[190–193] Fiber-shaped energy-storage devices can be combined with textile technology to power future portable and wearable electronics, which will facilitate the emergence of wearable electronics. iii) Printable flexible energy-storage devices will be a promising field in the near future. Recently, printing technologies have been booming all over the world because of their large-scale, good flexibility, and low-lost features. Prospective flexible displays and lighting, thin-film solar cells, radio frequency identification (RFID) tags, sensors, and so on, can be fabricated directly by printing technologies. Therefore, future effort regarding printable energy-storage devices that can provide large-scale, cheap production processes and high flexibility are needed. When printable energy-storage cells are combined with printable devices, it is exciting to predict that fully printable electronics will come true. iv) As mentioned, integrated systems based on energy-storage devices currently arouse great interest, and many achievements have been obtained, while seldom realizing full flexibility. Flexible integrated energy-storage systems composed of flexible power sources and flexible electronic devices could be a hopeful research direction. We believe that fully flexible integrated energy-storage systems will emerge as a development promoter that could expedite the advent of the smart, ubiquitous and flexible integrated systems.

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potential applications in electronic textiles where a wire structure is required.[186–188] Figure 16b shows the schematic diagram of the fiber-shaped self-charged SC using a DSSC in the process of charging and discharging. These integrated devices are usually composed of two units, whereas more devices need to be combined together in order to achieve diversified functions. Very recently, as demonstrated in Figure 16c,d,[189] Xie et al.[189] exhibited a self-powered electrochromic smart window with tunable transmittance driven by dye-sensitized solar cells, which also acts as a photocharged electrochromic supercapacitor with high areal capacitance and reversible color changes, which can be potentially applied in buildings, cars and displays. On the other hand, as energy-storage and power-supply devices, LIBs and SCs can also be integrated with other units to achieve self-powered systems. Lee el at. showed an all-in-one flexible LED system integrated with a bendable LIB wrapped with PDMS sheets, which exhibits excellent flexibility.[53] Hou et al. also demonstrated flexible photodetectors based on SnO2 cloth powered by flexible LIB and high-performance photodetectors based on CdSe nanowires integrated with a flexible in-plane SC.[37,78] Very recently, Wang et al. developed a fiberbased, flexible integrated system to simultaneously realize both flexible energy storage and flexible optoelectronic detection on a single fiber device.[149] In this integrated system, a fiber-shaped flexible ASC, fabricated using titanium wire/Co3O4 nanowires as the positive electrode and graphene coated on carbon fibers as the negative electrode, was used as the energy-storage and energy-supply device; graphene was also used as the lightsensitive material. By detecting the leakage current under light illumination, a flexible photodetecting device was obtained. In their work, an all-flexible integrated system was successfully realized. These integrated self-powered sensors can be operated without an external power source, dramatically decreasing the extra weight of the electrical power source, exhibiting significant importance for specific applications including large-area wireless environmental sensing, chemical and biosensing, and in situ medical-therapy monitoring.

Acknowledgements The authors acknowledge support from the National Natural Science Foundation (91123008, 61377033, 21273290), the 973 Program of China (2011CB933300), the Program for New Century Excellent Talents of the University in China (grant no. NCET-11–0179), the Fundamental Research Funds for the Central Universities (HUST: 2013NY013) and Wuhan Science and Technology Bureau (20122497). Received: February 26, 2014 Published online: June 10, 2014

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Flexible energy-storage devices: design consideration and recent progress.

Flexible energy-storage devices are attracting increasing attention as they show unique promising advantages, such as flexibility, shape diversity, li...
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