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1. Introduction

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

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 fl exible electronics. [ 5,6 ] Therefore, the trend in the next genera-tion of energy-storage-device development is to realize light, fl exible, and small units with shape-conformability, aesthetic diver-sity, and excellent mechanical properties. The key challenges in achieving fl exible energy-storage devices are the selection and design of bendable current collectors with good mechanical properties and the fabrication of fl exible electrode materials with a high capacity and excellent con-ductivity. To fulfi ll and ameliorate fl exible energy-storage devices, such as fl exible lithium-ion batteries and fl exible super-capacitors, tremendous effort has been made in recent years. Figure 1 shows sev-eral typical examples of designed fl exible energy-storage devices and their potential applications in fl exible electronics, print-

able electronics, wearable electronics, and integrated systems. This review focuses on the recent development on fl exible

energy-storage devices, including fl exible lithium-ion batteries (LIBs) and fl exible supercapacitors (SCs). In the fi rst part, we review the latest successful examples of fl exible LIBs based on conductive paper, three-dimensional electrodes, solid-state elec-trolytes, and novel structures, along with their technological innovations and challenges. In the following section, a detailed overview of the recent progress regarding fl exible SCs based on carbon materials and other composites coupled with fl exible micro-supercapacitors are given, combined with discussion of the future challenges and opportunities for the fabrication of the state-of-the-art fl exible supercapacitors. Finally, we briefl y introduce some of the latest achievements in the recently high-lighted integrated system based on energy-storage devices.

2. Flexible Lithium-ion Batteries

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

Flexible energy-storage devices are attracting increasing attention as they show unique promising advantages, such as fl exibility, shape diversity, light weight, and so on; these properties enable applications in portable, fl exible, 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 fulfi ll the requirements of future fl ex-ible energy-storage devices, and much progress has been witnessed. This review describes the most recent advances in fl exible energy-storage devices, including fl exible lithium-ion batteries and fl exible supercapacitors. The latest successful examples in fl exible lithium-ion batteries and their technological innovations and challenges are reviewed fi rst. This is followed by a detailed overview of the recent progress in fl exible supercapacitors based on carbon materials and a number of composites and fl exible micro-supercapacitors. Some of the latest achievements regarding interesting integrated energy-storage systems are also reviewed. Further research direction is also pro-posed to surpass existing technological bottle-necks and realize idealized fl exible energy-storage devices.

DOI: 10.1002/adma.201400910

X. Wang, [+] B. Liu, Prof. G. Shen State Key Laboratory for Superlattices and Microstructures Institution of SemiconductorsChinese 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 EngineeringSun Yat-sen University Guangzhou 510275 , PR China E-mail: [email protected] [+] These authors contributed equally to this work

Adv. Mater. 2014, DOI: 10.1002/adma.201400910

http://doi.wiley.com/10.1002/adma.201400910

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explored. It has been found that the fl exibility of functional devices are largely determined by the electrodes or the current collectors. [ 21 ] Thus, to fully realize fl exible LIBs, soft electrode-active 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 fl exible LIBs. Moreover, the electrochemical reactions of batteries are commonly determined by electron and ion transport at the surface of the electrodes. By using nano-structured 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 fl exible 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 cur-rently investigated because the surface roughness and porous structure of paper and textile are ideal for ions transportation and the fl exibility of the paper and textile benefi ts the realiza-tion of fully bendable batteries. However, conventional paper and textile substrates are not electrically conductive for elec-trons. Therefore, highly conductive paper for fl exible batteries is strongly needed. Cui et al. demonstrated that conformal coating of single-walled carbon nanotubes (SWNTs) on the sur-face 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 fl exible conductive paper

or textile will benefi t the investigation of paper-thin, fl exible, and lightweight LIBs.

Carbon nanotubes (CNTs) have been widely used as elec-trodes in electrochemical devices due to their extreme fl ex-ibility 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 signifi cant 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 fl exibility and could be rolled up, twisted, even bent to any degree and was com-pletely recoverable, which could be directly used as the fl exible electrode for paper battery. [ 11 ] Using the nanocomposite paper as cathode and a thin evaporated Li-metal layer as anode, the assembled fl exible 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 fl exible and easily dam-aged during the bending process.

Flexible paper batteries with conductive paper as the cur-rent collectors for both anode and cathode were later demon-strated. [ 15 ] During the fabrication process, no metal is used, which greatly benefi ts the realization of full fl exible paper bat-tery. As shown in Figure 2 a, highly conductive SWNT fi lms

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 electro-chemical synthesis of alloys,

intermetallic compounds and metal oxide nanomaterials, and investigation of their applications for energy conver-sion 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 fl exible electronics and printable electronics, including transis-tors, photodetectors, sensors, and fl exible energy-storage devices.


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

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were used as current collectors, and the active electrode mate-rials were coated on the surface of the SWNT fi lms. Figure 2 b and 2 c show the schematic illustration and the fi nal paper battery device before encapsulation and cell testing. It is the fi rst report of the use of commercial paper in LIBs, where paper is used as both separator and mechanical support. [ 15 ] Figure 2 d shows a highly fl exible, rechargeable paper battery lighting up an LED device. The fi rst-cycle voltage profi le of the paper battery is illustrated in Figure 2 e. 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 2 f. The coulombic effi ciency is 94–97% after the fi rst 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 fl exible paper battery renders it as candidate for fl exible elec-tronic 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. Strate-gies 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 strate-gies, 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 ]

To fabricate 3D fl exible electrodes, fl exible 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 ZnCo 2 O 4 nanowire arrays/carbon cloth hierarchical structure as an integrated electrode for fl ex-ible LIBs. [ 19 ] The obtained ZnCo 2 O 4 /carbon cloth composite with highly ordered 3D ZnCo 2 O 4 arrays could be grown on individual carbon microfi ber ( Figure 3 a). Using the com-posite as binder-free anode and LiCoO 2 /Al foil as cathode, fl exible full battery with operating window between 2.2 and 3.7 V was assembled. The as-fabricated fl exible battery exhib-ited high capacity of about 1300 mA h g −1 and kept about 96% of its initial value after 40 cycles (Figure 3 b). The pack-aged full battery can easily light an LCD mobile display even under bending state. The folding endurance is an important parameter to refl ect the fl exibility. No performance degradation was observed from the voltage profi les by bending the fl exible battery for hundreds cycles (Figure 3 c), revealing its excellent electrical and mechanical stability. Liu and co-workers also fab-ricated self-assembled ZnCo 2 O 4 urchins on carbon fi bers as the binder-free anodes for fl exible 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 fl exible batteries with similar structure were also demonstrated based on different anodes, such as stannic oxide, metal germinate, and silicon nanow-ires; [ 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 specifi c energy density of the full battery to some extent. Thus light-weight and fl exible electrodes or current collectors with 3D architectures are still a challenge. Very recently, Wang et al. fabricated a 3D web-like Li 4 Ti 5 O 12 architecture on an ultrathin titanium foil (Figure 3 d),


Figure 2. Schematic illustration of: a) lamination process and b) the fi nal 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 bat-tery (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.

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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 LiMn 2 O 4 nanorods on stainless steel cloth as cathode, fl exible full battery was successfully assembled with an discharge platform of about 2.4 V (Figure 3 e). 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 fl exibility (Figure 3 f). However, the use of metal substrates may also go against the realization of fully fl exible batteries.

Recently, a unique 3D graphene macroscopic structure: gra-phene foam (GF) was extensively studied in fl exible energy-storage devices due to the high conductivity, light weight, high specifi c surface area, and excellent fl exibility. [ 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 fl ux, benefi ting the fast charge and discharge capability of LIBs. By composting the GF with LiFePO 4 and Li 4 Ti 5 O 12 , Cheng et al. developed thin, light-weight, and fl exible LiFePO 4 /GF and

Li 4 Ti 5 O 12 /GF electrodes with high rate per-formance due to the contribution of GF. The hybrid materials exhibited outstanding fl ex-ibility and could be bent into arbitrary shapes without degradation. Excitingly, the 3D inter-connected network structure can also been completely preserved ( Figure 4 a,b). Figure 4 c shows the Li 4 Ti 5 O 12 nanosheets in the hybrid materials; these materials exhibit a long, fl at potential plateau at 1.0 V and a high capacity of 86 mA h g −1 at 200 C. Using the fl exible LiFePO 4 /GF and Li 4 Ti 5 O 12 /GF as electrodes, a thin, light-weight and fl exible full LIB with excellent electrochemical performance is demonstrated. The fl exible device can power a red-light-emitting diode (LED) even when bent revealing its high bendability (Figure 4 d). The assembled battery shows a stable operating voltage of 1.9 V even under bending state (Figure 4 e). The bendability is further investigated by comparing the per-formances under bent and fl at states. After 20 bends to a radius of 5 mm, the capacity of the as-assembled fl exible battery decreased less than 1% demonstrating the high fl ex-ibility and mechanical stability (Figure 4 f). 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, fl exible, and high-rate LIBs.

2.3. Flexible Lithium-Ion Batteries Based on Solid Electrolytes

To fabricate the fl exible batteries, liquid-type electrolytes have been mainly used. However, the use of liquid-type electrolytes may limit the realization of a really fl exible LIB because their thermal stability, mechanical stability and cell safety should be carefully considered in a fl exible battery based on liquid-type electrolytes. [ 20 ] To realize the full-fl edged fl exible batteries with high performance and robust safety, designing and synthesizing readily-deformable solid electrolytes are strongly desired to secure the safety of fl exible cells. [ 45–48 ] In the past several years, plastic crystal electrolytes (PCEs), com-posed 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 modifi ed 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 temper-ature, while the PCPEs still suffer from insuffi cient mechanical stability and are too thick, hindering their applications in fl ex-ible batteries with high safety.

To achieve the safety and mechanical stability of fl exible LIBs, Lee et al. developed a new class of highly thin, deformable, and


Figure 3. a) SEM images of the ZnCo 2 O 4 nanowire arrays on carbon cloth. b) Charge–discharge curves of the fl exible full battery. c) Flexibility of the full battery. a–c) Reproduced with permis-sion. [ 19 ] Copyright 2012, American Chemical Society. d) SEM image of the obtained Li 4 Ti 5 O 12 architecture. e) Charge–discharge curves at 2 C. f) Lighting an LED by the fl exible full battery. d,e,f) Reproduced with permission. [ 39 ] Copyright 2014, Springer.

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safety-reinforced N-PCPEs solid electrolyte. [ 20 ] In their work, a polyethylene terephthalate (PET) nanowoven is used to improve the mechanical properties of the N-PCPE. Through the in situ UV-crosslinking of ETPTA monomer under the co-presence of PCE, the 3D polymer electrolyte matrix can be formed directly inside the PET nanowoven skeleton. From Figure 5 a, one can found that the PET porous nanowoven is well-impregnated with the plastic crystal polymer electrolyte matrix. The ionic conduc-tivity of the N-PCPE shows no signifi cant decrease compared with the EC/DMC-based liquid electrolyte at high temperature of 80 °C (Figure 5 b) and high deformability. Using LiCoO 2 as

cathode, Li 2 Ti 5 O 12 as anode, and N-PEPC as both the electrolyte and separator mem-brane, fl exible battery maintained high safety even under serious shape deformations. As shown in Figure 5 c, the as-fabricated fl ex-ible 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 5 d). These results indi-cate that the N-PCPE is an advanced solid electrolyte for fl exible LIBs with high safety and outstanding fl exibility.

The thickness of the as-fabricated N-PCPE is about 25 µm, much thinner than that of conventional PCPE fi lm (more than 100 µm) and X-PCPE fi lm (about 210 µm). However, it still cannot meet the demands for light-weight and thin-fi lm shape of fl exible LIBs required for nano/microelectromechnical systems at the nanoscale or microscale.

Lithium phosphorus oxynitride electrolyte (LiPON), physically deposited as amorphous thin fi lm electrolytes, has been recently explored as electrolyte in all-solid-state microbatteries. [ 51–53 ] Using the physically deposited LiPON as both a solid electrolyte and separator, a thin-fi lm fl exible LIB based on all-solid-state materials has been demonstrated. [ 53 ] The fl exible LIB shows high robustness by turning on a blue LED under the bending state ( Figure 6 a). The cross-sectional structure of the solid-state LIB on a mica substrate can be seen from Figure 6 b, where we can fi nd that the thickness of the LiPON electrolyte is only about 2 µm, so thin that it can be widely used in all-solid-state,

ultrathin, and fl exible microbatteries. At var-ious bending radius values, the performance of the battery did not show any external damage. With increasing the bending curva-ture, the specifi c capacity of 106 µA h cm −2 for non-bending state is gradually decreased to 99 µA h cm −2 at R c = 3.1 mm, and the capacity loss is negligible during 100 charge and discharge cycles (Figure 6 c). 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 pro-cesses. No voltage change of the battery at bending condition reveals the mechanical stability of the fl exible thin-shaped battery (Figure 6 d). 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 fl exible integrated electronic sys-tems. However, the solid state electrolytes suffer from low ionic conductivity, especially at low temperatures, which needs to be fur-ther resolved through exploiting new solid-state electrolyte materials with higher ionic conductivity at room/low temperature.


Figure 4. a) Photograph of a free-standing fl exible LTO/GF bending bent. b,c) SEM images of the LTO/GF. d) Photograph of a bent battery lighting a red LED device. e) Galvanostatic charge–discharge curves of the battery. f) Cyclic performance of the battery under fl at and bent states. Reproduced with permission. [ 18 ] Copyright 2012, National Academy of Sciences.

Figure 5. a,b) Cross-sectional SEM image (a) and thermal stability of the N-PCPE (b). c) Pho-tograph showing active state of a red LED lamp connected to the wrinkled cell. d) Charge–dis-charge profi les of the wrinkled cell with cycling (charge/discharge current densities = 0.2 C/0.2 C). Reproduced with permission. [ 20 ] Copyright 2013, Wiley.

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2.4. Structural Design of Flexible Lithium-Ion Battery

In portable electronics, the shape of the battery is a particular limiting factor for the creation of practical and aesthetic devices. Indeed, a groundbreaking technology to design novel 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 fl exible batteries with alterative shapes. However, the large voltage, the struc-tural limitations of the planar architecture, and the lack of desired omi-directional fl ex-ibility 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 archi-tecture, cable-type LIBs might indeed provide the breakthrough necessary in fl exible elec-tronics because of their excellent mechanical fl exibility. Recently, LG Chem. Ltd demon-strated a cable-type LIB [ 57,58 ] characterized by a hollow spiral, spring-like anode (com-prising nickel/tin-coated copper wires) and a modifi ed polyethylene terephthalate (PET) nanowoven separator membrane, and the cross-sectional structure of the cable bat-tery is shown in Figure 7 a. The cable battery exhibited a potential plateau at about 3.5 V and a specifi c capacity of 1.0 mA h cm −1 for

the fi rst charge/discharge cycle (Figure 7 b). Mechanical bending test (Figure 7 c) reveals that the voltage of the battery could be stably maintained when the battery is bent to dif-ferent states. A photograph of the cable LIB with excellent mechanical fl exibility is shown in Figure 7 d. The wire shape and omni-directional fl exibility 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 fi t nearly anywhere, thereby facilitating the realization of practical wearable electronics.

Along with the fast development of fl ex-ible electronics, stretchable electronics are springing up as a new technological advancement due to their reversible stretch-ability while still maintain their function-ality. Stretchability represents a rigorous challenging of mechanical stability. The stretchable devices must bear large strain deformation, and large shape deformation, including not only bending, but also twisting, stretching, compressing and others. There-fore, stretchable energy-storage devices are

strongly desired to power the stretchable electronics. [ 59 ] In 2013, Rogers et al. introduced a high stretchable LIB with the active materials segmented design, and unusual ‘self-similar’ interconnect structures between them. [ 60 ] The stretchable LIB was fabricated on a thin silicone elastomers as substrates,


Figure 6. a) Photograph of an LED powered by a fl exible LIB under bending state. b) Cross-section of the LIBs on a mica substrate. c) Capacity retention of the fl exible LIBs after 100 cycles under different states. d) Voltage retention during one bending cycle. Reproduced with permission. [ 53 ] Copyright 2012, American Chemical Society.

Figure 7. a,b) Side view (a) and fi rst charge–discharge–profi les (b) of the cable battery. c) Dis-charge characteristics with variations in bending strain. d) Photograph of the prototype cable battery used to power a red LED screen. Reproduced with permission. [ 57 ] Copyright 2012, Wiley.

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as illustrated in Figure 8 a,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 8 c). 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 8 d. Surprisingly, the stretchable battery could turn on a commercial LED even when it was stretched by up to 300%. Figure 8 e and 8 f show the high stretchability of the battery which satisfi es the requirements for many applications that are being contemplated for stretchable electronics.

3. Flexible Supercapacitors

Supercapacitors (SCs), also known as electrochemical capaci-tors or ultracapacitors, have attracted considerable interest in recent years because of their relatively high energy den-sity, 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 capaci-tors, 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

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

3.1. Flexible SCs Based on Pure Carbon Materials

Over the past few decades, pure carbon materials bring sub-stantial opportunities for developing advanced fl exible elec-trodes given that their high conductivity, high surface area (up to 2630 m 2 g −1 ), lightweight, high temperature stability, control-lable 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 devel-opments of fl exible 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 fl exible SCs, due to its high specifi c surface area (1240–2200 m 2 g −1 ), high electrical con-ductivity (10 4 –10 5 S cm −1 ), and controllable regular pore struc-ture. [ 89,105–108 ] Moreover, CNTs possess a high aspect ratio, which not only provides long continuous conductive paths, but also ensures high fl exibility. [ 89 ] Kaempgen et al. reported print-able thin-fi lm 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 dis-connected or missing links within the network. Consequently, the as assembled fl exible SC device exhibited an energy density of 6 W h kg −1 in H 3 PO 4 /Polyvinyl alcohol (PVA) gel electro-lytes. However, the smooth fl at 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 fi lm delaminating was observed. To increase CNT mass loading, Hu et al. prepared fl exible paper-based electrodes by coating conductive SWCNT


Figure 8. a,b) Schematic illustration (a) and exploded view layout (b) of a completed battery. c) Galvanostatic charging and discharging of the bat-tery 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.

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printing paper pre-treated by polyvinylidene fl uoride (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 fl exibility.

Recently, bacterial nanocellulose (BNC) papers were also used as substrates to fab-ricate fl exible CNT electrodes. Kang et al. reported a vacuum-fi ltering 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 fl exible all-solid-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 sim-ilar fl exible SCs with a voltage of 3 V was also developed by Kang et al. [ 89 ] They coated CNTs on offi ce papers as fl exible electrodes and used an ionic-liquid-based gel as elec-trolyte to assemble SCs. The as-fabricated solid-state SCs exhibited excellent electro-chemical properties, stability and fl exibility, with the maximum energy density of 41 W h kg −1 and power density of 164 kW kg −1 . Zheng and co-workers prepared a kind of SSC device based on the TiO 2 @C core–shell nanowires on carbon cloth and PVA/H 2 SO 4 gel electrolyte, which exhibits an excellent fl exibility that can even be folded and twisted without destroying their electrochemical properties, and a maximum energy density of 0.011 mW h cm −3 was obtained. [ 109 ] Recently, Niu et al. synthesized highly stretchable buckled SWCNT fi lms by combining directly grown SWCNT fi lms with polydimethyl-siloxane (PDMS), and they reported their implementation as electrodes in fl exible SCs. [ 110 ] The SC device based on the buckled SWCNT fi lms possessed excellent fl exibility, and exhib-ited remarkably stretchable, whose performance showed littile degradation when stretched under 120% strain ( Figure 9 a,b).

3.1.2. Flexible Graphene-Based SCs

Graphene, which consists of single or few stacked ordered sp 2 carbon sheets, have attracted substantial interest in fl exible SCs owing to its high electrical conductivity, and specifi c sur-face area (up to 2630 m 2 g −1 ). [ 62,99,111–113 ] Yoo et al. developed ultrathin fl exible SCs based on the composites of pristine graphene and multilayer reduced graphene oxide (RGO) and PVA/H 3 PO 4 gel electrolyte. [ 114 ] These fl exible SCs made from 1–2 graphene layers achieved a specifi c capacitance of up to 80 µF cm −2 and could further improved after using more gra-phene layers. Choi et al. also fabricated a fl exible thin SC device

by integrating the robust, conductive, and free-standing Nafi on-functionalized reduced graphene oxide (f-RGO) electrodes and solvent-cast Nafi on electrolyte membranes. [ 115 ] The f-RGO-based SCs showed a higher specifi c capacitance (118.5 F g −1 at 1 A g −1 ), anout two times of the pristine RGO-based SSCs (62.3 F g −1 at 1 A g −1 ), showing the Nafi on-functionalization can signifi cantly improve the electrochemical performance of the graphene. However, its capacitive properties of graphene such as capacitance, energy density, and power density have remained lower than expected, [ 97,116 ] which can be ascribed to the restacking of graphene sheets during its processing due to the strong sheet-to-sheet van der Waals interactions. [ 62 ]

Up to now, considerable effort has been dedicated to 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 fi lms and the use of these unrestacked graphene paper as high-performance electrodes for fl exible SCs. [ 99 ] These face-to-face-stacked multilayered graphene sheets possessed a highly open pore structure with large specifi c surface area, which allows the electrolyte solution easily access to the sur-face of individual sheets. As a result, fl exible SCs based on these self-stacked, solvated graphene paper electrode exhibited a high specifi c capacitance of 273.1 F g −1 and a substantially high energy density up to 150.9 W h kg −1 . El-Kady et al. recently developed a standard Light Scribe DVD optical drive to do the direct laser reduction of graphite oxide (LSG) fi lms to graphene and demonstrated their applications as fl exible electrodes in


Figure 9. a) CV curves of the stretchable SC based on buckled SWCNT fi lms under 120% strain at different scan rates. b) The specifi c capacitances of a stretchable SC under different strains. a,b) Reproduced with permission. [ 110 ] Copyright 2012, Wiley. c,d) SEM image of the 3D graphene hydrogel (c), and CV curves of the fl exible solid-state device at 10 mV/s for different bending angles (d). c,d) Reproduced with permission. [ 44 ] Copyright 2013, American Chemical Society.

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fl exible SCs. [ 62 ] The LSG has an extremely large specifi c surface area of 1520 m 2 g −1 and high conductivity of 1738 S m −1 . Flex-ible SCs based on these LSG electrodes and an ionic liquid elec-trolyte 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-perfor-mance electrodes in fl exible SCs. Electrically conductive, light-weight and paper-like rGO foams obtained by autoclaved leav-ening and steaming of GO layered fi lms achieved a specifi c capacitance of 110 F g −1 and excellent fl exibility. The synthesis of graphene hydrogel is another key method for designing 3D structure. Xu et al. [ 44 ] recently fabricated fl exible solid-state SCs based on 3D graphene hydrogel fi lms, as shown in Figure 9 c. With a highly interconnected 3D network structure, the as-synthesized graphene hydrogel exhibited exceptional electrical conductivity (192 S m −1 ), specifi c surface area (ca. 414 m 2 g −1 ) and mechanical robustness. All these features make it very promising for fl exible SCs. The as-assembled solid-state SC device using these 3D graphene hydrogel yielded outstanding capacitive performance in H 2 SO 4 /PVA gel electrolyte, such as a high specifi c capacitance of ca. 186 F g −1 (1 A g −1 ), good sta-bility, high energy density of about 6.5 W h kg −1 and excellent fl exibility (Figure 9 d). They also fabricated fl exible SC device by using the functionalized 3D graphene hydrogels, which exhibited enhanced specifi c 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 solid-state ionic liquid electrolyte can also improve the energy den-sity of graphene-based SCs. Tamilarasan et al. [ 96 ] reported a mechanically stable, fl exible 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 fl exible SCs achieved a high energy density of 32.3 W h kg −1 .

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 fl exibility compared with each single constituent component. [ 103,104 ] Very recently, Cheng et al. [ 117 ] synthesized 1D CNTs on 2D graphene (CNTs/G fi ber). The fl exible textile of CNT/G fi bers was used as electrodes for construction of fl ex-ible SCs. Due to the high surface area, higher mechanical fl ex-ibility and electrical conductivity of CNT/G fi ber electrode, the as assembled SCs displayed an outstanding capacitive perfor-mance 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

order to increase the energy density of fl exible 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 specifi c 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 fl exible SCs based on the composited electrodes.

To push the energy-density limit of fl exible SCs, a great deal of effort has been devoted to improving the capacitance by fab-ricating composited electrodes. In particularly, composites con-sist of carbon materials and pseudocapacitive materials such as MnO 2 , [ 103,106,118,122,126 ] RuO 2 , [ 127 ] polyaniline (PANI) [ 121,128–130 ] are the most promising and studied electrode materials for fl ex-ible SCs. This kind of composite can effectively take advantage of both the high conductivity of the carbon materials and the high specifi c capacitance of the pseudocapacitive materials, and hence result in signifi cant improvements in both the energy density and the power density.

Among metal oxides, MnO 2 has been widely recognized as one of the most attractive electrode materials for SCs in terms of its high theoretical specifi c capacitance (ca. 1400 F g −1 ), low cost, natural abundance, and environmental compatibility. [ 131 ] In recent years, the fabrication of fl exible SCs based on MnO 2 electrodes have become a hot research spot. [ 131,132 ] For instance, a fl exible solid-state SC device with an energy utiliza-tion effi ciency of about 80% has been reported by using MnO 2 nanorods grown on carbon cloth as electrodes. [ 132 ] However, the energy density of the fl exible SCs based on pristine MnO 2 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 fl exible MnO 2 -based SCs, hybrid MnO 2 /carbon com-posites such as carbon nanoparticles (CNPs)/MnO 2 , [ 122 ] CNTs/MnO 2 , [ 106,118 ] graphene/MnO 2 [ 126,133 ] and graphene/MnO 2 /CNTs [ 103 ] have been actively explored. MnO 2 was usually coated onto the surface of carbon materials via various methods. For example, Yuan et al. electrodeposited MnO 2 onto the CNPs sur-face and demonstrated a high-performance fl exible solid-state SC device with these CNPs/MnO 2 electrodes and a PVA/H 3 PO 4 gel electrolyte. [ 122 ] The device has good electrochemical perfor-mances 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 fl exible aqueous SC device made from MnO 2 -CNTs-cotton electrodes achieved high areal capacitance of 0.48 F cm −2 and high specifi c energy density of 20 W h kg −1 . [ 118 ] He et al. developed a freestanding 3D graphene/MnO 2 composite networks as electrodes for fl exible aqueous SCs. [ 133 ] The freestanding, lightweight 3D graphene networks was prepared from the pressed Ni foam, exhibiting superior mechanical strength and fl exibility. A composite 3D electrode could be obtained by coating a large and uniform mass of MnO 2 onto the entire skeleton by using electrodeposition. The as-assembled fl exible SC is demonstrated in Figure 10 a. The device based on the 3D graphene/MnO 2 composite electrodes showed high specifi c capacitance, excellent rate capability, and


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long cycling performance. Moreover importantly, the fabricated fl exible SCs based on these 3D graphene/MnO 2 composite elec-trodes were able to operate in a voltage of 1 V and achieve an energy density of 6.8 W h kg −1 at a power density of 62 W kg −1 (Figure 10 b). Similarly, Peng and co-workers also developed a fl exible, high-performance, in-plane SC based on MnO 2 /graphene composites. [ 126 ] This planar solid-state SC delivered a high specifi c capacitance of 267 F g −1 at current density of 0.2 A g −1 and excellent rate capability and cycling performance with capacitance retention of 92% after 7000 cycles. Recently, a kind of fl exible aqueous SC device based on interconnected gra-phene/MnO 2 /CNTs nanocomposite electrode has been dem-onstrated to possess ultrahigh performance by Cheng et al. [ 103 ] The synergistic effects from graphene, CNTs, and MnO 2 ena-bles effi cient charge transport and electrode integrity, endowing the fi lms with outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that were not achieved by any of these components alone. The SC device assembled from the graphene/MnO 2 /CNTs nanocom-posite electrode delivered remarkable specifi c capacitance of 70 F g −1 at 10 mV s −1 and high energy density of 8.9 W h kg −1 at 106 W kg −1 .

Hybrid composites of carbon materials and conducting polymers have been also extensively studied in fl exible SCs. [ 74,121,128–130 ] Meng et al. developed an ultrathin all-solid-state paper-like polymer supercapacitor with a total thickness of 100 µm. [ 121 ] The device was assembled by solidifying two pieces of PANI/CNT composite fi lms (Figure 10 c) in a PVA/H 2 SO 4 gel electrolyte. The maximum energy density and power density of this device achieved are 7.1 W h kg −1 and 2189 W kg −1 (based

on the whole device, Figure 10 d), respec-tively. A similar fl exible SC based on PANI/single-walled carbon nanotubes (SWCNTs) composite electrode was designed by Wang et al. [ 128 ] and exhibited a high energy den-sity of 26.6 W h kg −1 (based on electrode). To meet the development of transparent and fl exible optoelectronic devices, Ge and co-workers used a kind of PANI/SWCNTs composite electrode to develop a transparent and fl exible solid-state SC. [ 129 ] This trans-parent SC showed a specifi c 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 fl exible SC based on PANI/multiwalled carbon nanotubes (MWCNTs) composite fi lms. [ 130 ] The PANI/MWCNTs fi lms were synthesized via a simple electro-deposition process and achieved a maximum specifi c capacitance of 233 F g −1 at a current density of 1 A g −1 .

Metal nitrides such as titanium nitride (TiN) [ 76 ] and vanadium nitride (VN) [ 125 ] have received increasing attention as electrode materials for fl exible SCs. Lu et al. recently reported a high-performance solid-state SC device based on the stabilized TiN nanowire electrode for the fi rst time. [ 76 ] The fabricated TiN-based SC device achieved an excellent

volumetric specifi c capacitance of 0.33 F cm −3 and a maximum energy density of 0.05 mW h cm −3 . In addition, this device also exhibited outstanding cycling performance that could retain 83% of its initial capacitance after 15 000 cycles. In order to further enhance the energy density of metal nitrides, Xiao et al. synthesized a hybrid VN/CNTs composite electrode by a simple vacuum-fi ltering method. [ 125 ] The areal capacitance of this hybrid VN/CNTs electrode reached 178 mF cm −2 at current density of 1.1 mA cm −2 . When using the hybrid VN/CNTs elec-trodes to assemble a solid-state SC, the fabricated SC was able to operate in a voltage of 0.7 V and has good mechanical fl ex-ibility. Furthermore, this device achieved a high volume capaci-tance of 7.9 F cm −3 and energy density of 0.54 mW h cm −3 .

In addition to the composites of carbon materials and pseu-docapacitive materials, other composites of pseudocapacitive materials with other conductive materials such as Au, [ 120,134 ] WO x , [ 123 ] ZnO, [ 124 ] TiO 2 , [ 135 ] CoAl-layered double hydroxide [ 119 ] and conducting polymer such as polypyrrole (PPy) [ 136 ] have been explored to fabricate fl exible SCs.

To improve the electric conductivity of MnO 2 , Lang et al. used nanoporous Au as highly conductive material to sup-port MnO 2 . [ 134 ] The nanoporous Au enables the fast electrons transport through the MnO 2 , and can greatly facilitate the ion diffusion between the MnO 2 and the electrolytes, and thus exhibiting an ultrahigh specifi c capacitance of 1145 F g −1 . Addi-tionally, the energy and power densities of fl exible SCs based on this kind of hybrid Au/MnO 2 electrode increased with the loading amount of MnO 2 , and achieved maxima of 57 W h kg −1 and 16 kW kg −1 , respectively. Recently, Lu et al. also demon-strated a kind of high-performance fl exible SCs by using hybrid


Figure 10. a) Schematic diagram and photoimages of fl exible symmetrical SCs based on gra-phene/MnO 2 composite electrodes. b) CVs of the fl exible supercapacitors at scan rates from 50 to 1000 mV s −1 . a,b) Reproduced with permission. [ 133 ] Copyright 2013, American Chemical Society. c) SEM image of the PANI/CNT composite electrode. d) Ragone plots for the electrode materials and for the entire all-solid-state device. c,d) Reproduced with permission. [ 121 ] Copy-right 2010, American Chemical Society.

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WO 3– x @Au nanowires as highly conductive core to support MnO 2 . [ 123 ] Amorphous MnO 2 was electrodeposited onto the surface of WO 3– x @Au nanowires grown on carbon cloth, as demonstrated in Figure 11 a and b. These WO 3– x @Au@MnO 2 core–shell nanowires achieved an ultrahigh specifi c capacitance of 1195 F g −1 at current density of 0.75 A g −1 with outstanding long-term cycling performance. Moreover, these WO 3– x @Au@MnO 2 core–shell nanowires have a remarkable energy density of 106.4 W h kg −1 at power density of 23.6 kW kg −1 and a high power density of 30.6 kW kg −1 at energy density of 78.1 W h kg −1 . Additionally, the solid-state SCs assembled by two WO 3– x @Au@MnO 2 nanowire electrodes and PVA/H 3 PO 4 gel electrolyte represent a high fl exibility that can even endure folding and twisting without affecting their performances (Figure 11 c). Similarly, Yang et al. electrodeposited amorphous MnO 2 onto hydrogenated ZnO (denoted as H-ZnO) nanowires grown on carbon cloth to obtain H-ZnO@MnO 2 core–shell nanowires (Figure 11 d). [ 124 ] These H-ZnO@MnO 2 nanowires showed excellent electrochemical performance with a high areal capacitance of 138.7 mF cm −2 (1260.9 F g −1 ). By using these H-ZnO@MnO 2 nanowires as electrodes and PVA/LiCl gel as electrolyte, the fabricated fl exible solid-state SCs exhib-ited an areal capacitance of 26 mF cm −2 , good cycling perfor-mance with 87.5% retention of the original capacitance after

10 000 cycles, and high fl exibility with little performance degradation even under bent and twisted states (Figure 11 e).

Conducting-polymer-based composites have also been investigated as high-perfor-mance electrode materials for fl exible asym-metric SCs. [ 120 ] For example, an ultrathin all-solid-state fl exible SC device with a thick-ness of less than 1 µm, based on the nano-porous Au/PPy composite was reported by Meng and co-workers. [ 120 ] Nanoporous Au/PPy composites were obtained by a conven-ient dealloying and electropolymerization process. The overall thickness of the as-fab-ricated SCs based on these Au/PPy compos-ites was only about 600 nm, which is typi-cally one or two orders of magnitude thinner than other reported fl exible SCs. Due to the fast responses to ions and electrons, this device was able to deliver an ultrahigh volu-metric energy density of 2.8 mW h cm −3 and power density of 56.7 W cm −3 . Furthermore, this device also exhibited extraordinary fl ex-ibility and cycling stability. Yu et al. recently reported the facile synthesis and improved electrochemical performance of the TiO 2 @PPy core–shell nanowires. [ 135 ] Electrochem-ical measurements show that the TiO 2 @PPy core–shell nanowires on carbon cloth exhib-ited a high areal capacitance of 64.6 mF cm −2 at a scan rate of 10 mV s −1 . Moreover, the direct growth of nanowires on carbon cloth render the integrated electrode a high surface area, and can be directly used as the elec-trode for fl exible SC without any additives.

The solid-state SC device based on the as-prepared TiO 2 @PPy nanowire electrodes exhibited good fl exibility and achieved a maximum energy density of 0.013 mW h cm −3 . Very recently, a kind of conductive PPy/paper has been successfully synthe-sized by a simple “soak and polymerization” and shows great potential as fl exible electrode for solid-state SCs. [ 136 ] The fabri-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 weight of the fl exible solid-state SCs assembled with the PPy/paper composite electrodes was only about 55 mg, and could operate in a voltage of 0.8 V. This SC device was able to achieve an areal capacitance of 0.42 F cm −2 and a high energy density of 1 mW h cm −3 at power density of 0.27 W cm −3 . In addi-tion, they also developed a kind of fl exible PANI-based SCs by using Au/paper as a conductive substrate to support PANI. [ 137 ] This PANI-based SCs was also able to operate in a voltage of 0.8 V and exhibited a higher volumetric energy density of 10 mW h cm −3 at power density of around 3 W cm −3 .

Layered double hydroxides (LDHs) are promising pseudo-capacitive materials owing to their high redox activity, low cost and environmentally friendly nature. On the other hand, as a derivative of polythiophene, poly(3,4-ethylenedioxythiophene) (PEDOT) has received considerable attention as an electrode material due to its large electroactive potential window and


Figure 11. a) Schematic of the fabrication process for WO 3– x @Au@MnO 2 NWs. b) SEM image of the products. c) Optical photograph of thefl exible SC. a–c) Reproduced with permission. [ 123 ] Copyright 2012, Wiley. d) SEM image of the H-ZnO@MnO 2 nanowires. e) CV curves of the SC collected under different bent states at a scan rate of 100 mV s −1 . d,e) Reproduced with permission. [ 124 ] Copyright 2013, American Chemical Society.

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et al. reported the use of CoAl-LDH nanoplatelets grown on fl exible Ni foil substrate as core to support electrochemically active PEDOT and demonstrated their prominent performance in fl exible SCs. [ 119 ] The LDH@PEDOT core–shell nanoplate-lets showed a remarkable specifi c 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 long-term 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 fl exible SCs can also be increased by maximizing the operation voltage. For this purpose, fl exible SCs based on organic electrolyte or ionic-liquid-electrolytes have attracted considerable attention since the organic electrolytes or ionic-liquid-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 sym-metric supercapacitors (SSCs), ASCs are able to be operated in much wider potential windows. [ 63,67,73,82,138–140 ] ASCs usu-ally 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 effec-tively 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 signifi cantly enhancing the device energy density. In this regard, fl exible 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 fl exible ASCs.

Transition metal oxides and Carbon materials are commonly employed as cathodes and anodes in fl exible ASCs due to their complementary working potential windows. [ 73,77,81,83,140,143 ] In particular, the combination of a MnO 2 cathode and a carbon anode is the most attractive and used system in fl ex-ible ASCs due to the high theoretical specifi c capacitance, low cost, abundance and environmental friendly nature of MnO 2 . [ 73,82,140,144,145 ] To date, a variety of fl exible ASCs based on the MnO 2 cathode and carbon-based anode have been devel-oped and this kind of fl exible 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 MnO 2 /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 Na 2 SO 4 and achieve a max-imum energy density of 12.5 W h kg −1 with excellent cycling stability. [ 144 ] Similarly, Sumboja et al. prepared a kind of MnO 2 /reduced graphene oxide (RGO) paper electrode with large areal mass of MnO 2 , and assembled an ASC device based on the RGO/MnO 2 paper as cathode, RGO paper as anode and 1 M

Na 2 SO 4 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 signifi cant advancement in the fabrication of thicker and large mass fl exible electrodes without sacri-fi cing their electrochemical performance. Shao et al. fabri-cated an aqueous fl exible ASC device with a voltage of 1.8 V by using a graphene/MnO 2 nanorod thin fi lm as the cathode and a graphene/Ag thin fi lm 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 den-sity was 24.5 kW kg −1 at an energy density of 12.3 W h kg −1 , respectively. Recently, fl exible 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/MnO 2 -based ASC device exhibited the max-imum 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 MnO 2 -PEDOT:PSS cathode and an active carbon anode can deliver a maximum volumetric energy density and power den-sity of 1.8 × 10 −3 W h cm −3 and 0.38 W cm −3 . [ 141 ]

The achievement of gel polymer electrolytes enables the fab-rication of fl exible quasi-solid-state/solid-state ASCs. In recent years, a number of different kinds of fl exible solid-state ASCs based on MnO 2 cathodes have also been developed. [ 73,81,139,146 ] Lu et al. reported the fi rst fabrication of high-performance fl ex-ible solid-state ASCs based on a hydrogenated TiO 2 (denoted as H-TiO 2 )@MnO 2 core–shell nanowire cathode and an H-TiO 2 @C core–shell nanowire anode. [ 73 ] H-TiO 2 nanowires grown on carbon cloth served as the conductive core to support amorphous MnO 2 and carbon shells and exhibited enhanced electrochemical performances. The stable operation voltage of as-fabricated solid-state ASC device using H-TiO 2 @MnO 2 nanowires as cathode and H-TiO 2 @C nanowires as anode in a PVA/ LiCl gel electrolyte can be extended to 1.8 V ( Figure 12 a). The volumetric and specifi c capacitances of the solid-state ASC device are comparable to the aqueous device and achieved a remarkable volumetric and specifi c capacitance of 0.71 F cm −3 and 141.8 F g −1 at 10 mV s −2 (Figure 12 b). The maximum vol-umetric 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 fl exibility that can be folded and twisted without losing its electrochemical perfor-mance. Gao et al. designed a kind of fl exible solid-state ASCs based on free-standing carbon nanotube/graphene and Mn 3 O 4 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@MnO 2 cathode, RGO anode and PVA/LiCl gel electro-lyte to develop a fl exible 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 MnO 2 //carbon system, other systems such as RuO 2 //graphene, [ 77 ] PANI//WO x @MoO x , [ 63 ] VO x //VN [ 67 ] and


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MnO 2 /PEDOT//PEDOT [ 148 ] have also been explored in con-structing high-performance fl exible ASCs. For example, Choi and co-workers developed a fl exible all-solid-state ASC device using an ionic liquid functionalized chemically modifi ed gra-phene (IL-CMG) fi lm anode, a hydrous RuO 2 -ILCMG com-posite cathode and a PVA/H 2 SO 4 gel electrolyte. [ 77 ] This opti-mized ASC device was able to operate in a voltage of up to 1.8 V and delivered a high energy density of 19.7 W h kg −1 . Moreover, this ASC device processed outstanding rate capability that can be operated even under an extremely high rate of 10 A g −1 with 79.4% retention of specifi c capacitance.

To further improve the energy density limit of fl exible ASCs, great effort has been devoted to exploring new anode with high capacitance and high conductivity. [ 63,67,148 ] Molybdenum oxide (MoO x ) has substantially higher capacitance than carbon-based materials but suffers from poor conductivity. To solve this problem, Xiao et al. reported the use of oxygen-defi cient tung-sten oxide (WO x ) nanowires as a scaffold to load electrochemi-cally active MoO 3- x , and demonstrated their superior perfor-mance as anode in fl exile solid-state ASCs. [ 63 ] The ASC device with the as-synthesized WO x @MoO x nanowires as anode, PANI nanowires as cathode, and PVA/H 3 PO 4 gel as the elec-trolyte could operate in a stable voltage of 1.9 V with exhibits a high areal capacitance of 216 mF cm −2 and a maximum energy density of 1.9 W h cm −3 at the power density of 0.035 W cm −3 .

Owing to its large specifi c capacitance (1340 F g −1 ), high elec-trical conductivity and suitable working window, vanadium nitride (VN) have been considered as a promise anode for ASCs. A new kind of solid-state ASC device with high energy density based on porous VN nanowire anode was recently demonstrated by Lu

et al. [ 64 ] These porous VN nano wires grown on carbon cloth (Figure 12 c) exhibited an excellent specifi c 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 signifi cantly improve the stability of VN nanowires without sacrifi cing their electro-chemical performance. Electrochemical studies showed that the ASC device using these stabi-lized VN nanowire as anode and VO x nanow-ires as cathode (Figure 12 c) was able to operate in a potential voltage window between 0 and 1.8 V (Figure 12 d) with 87.5% retention of ini-tial capacitance after 10000 cycles. A remark-able volumetric capacitance of 1.35 F cm −3 and specifi c capacitance of 60.1 F g −1 were achieved by this VO x //VN-ASC device at cur-rent density of 0.5 mA cm −2 . Additionally, the VO x //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, the ASC device achieved a maximum energy density of 0.61 mW h cm −3 at 0.5 mA cm −2 and a high power density of 0.85 W cm −3 at 5 mA cm −2 . These values are substantially higher than most of the reported quasi/all-solid-state SC devices.

Recently, Xu et al. developed a new kind of fl exible ASC with cobalt sulfi des (Co 9 S 8 ) as the anode and Co 3 O 4 /RuO 2 nanocomposites as the cathode. [ 142 ] Co 9 S 8 nanorod arrays were prepared on a carbon cloth by a hydrothermal sul-furation treatment of acicular Co 3 O 4 nanorod arrays, and the RuO 2 was directly deposited on the Co 3 O 4 nanorod arrays. Given that the Co 9 S 8 and Co 3 O 4 /RuO 2 electrodes have stable voltage windows between −0.3 and 0.6 V and between −1.0 and 0 V (vs SCE), respectively. The stable electrochemical win-dows of the Co 3 O 4 /RuO 2 //Co 9 S 8 -ASC device in both 3 M KOH aqueous electrolyte and PVA/KOH electrolyte can be extended to 1.6 V. Electrochemical results show that the volumetric capac-itance of the optimized ASCs achieved as high as 3.42 F cm −3 in aqueous electrolyte and 4.28 F cm −3 in PVA/KOH electrolyte at a current density of 2.5 mA cm −2 ( Figure 13 a). Additionally, 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 and 1.44 mW h cm −3 at 0.89 W cm −3 , respectively. Further-more, the as-fabricated solid-state Co 3 O 4 /RuO 2 //Co 9 S 8 -ASC device possessed remarkable fl exibility as the shape of cyclc voltammetry (CV) curves retained unchanged under normal, bent, and twisted conditions (Figure 13 b). Using the Co 3 O 4 nanowires on nickel fi ber (Figure 13 c) with operating window between 0.0 and 0.5 V as the positive electrode and graphene coated on carbon fi bers with voltage window from −1.0 to –0.3 V as the negative electrode, Wang et al. fabricated an all-solid-state fi ber-based fl exible ASC device, which can be oper-ated up to 1.5 V (Figure 13 d). [ 149 ] The volumetric capacitance of the device increased gradually from 0.234 to 0.695 F cm −3 with the potential increased from 0.6 to 1.5 V, thus the energy density can be increased at least by 1860%. The volumetric


Figure 12. a) CV curves of the solid-state H-TiO 2 @MnO 2 //H-TiO 2 @C-ASC device collected in different scan voltage windows. b) Volumetric and specifi c capacitance of the device calculated from the CV curves as a function of scan rate. a,b) Reproduced with permission. [ 73 ] Copyright 2013, Wiley. c) Photographs of carbon cloth substrates coated with VO x and VN nanowires. d) CV curves collected for VN and VO x nanowire electrodes at a scan rate of 10 mV s −1 . c,d) Reproduced with permission. [ 64 ] Copyright 2013, American Chemical Society.

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capacitance based on the total volume of the device reached 2.1 F cm −3 at 20 mA cm −3 , and maximum volumetric energy density of 0.62 mW h cm −3 and power density of 1.47 W cm −3 were also achieved.

3.4. Flexible Micro-SCs

The recent rapid advance and emergency of miniaturized, port-able consumer electronics stimulate the development of micro-scale power sources with high power density, towards the trend of being small, thin, lightweight, fl exible, and even wearable, to meet the growing demands of modern society. [ 150–152 ] Con-ventional SCs, however, are too large for very small devices and conventional manufacturing methods are not compatible with microelectronic fabrication. [ 153 ] As one type of newly developed miniaturized electrochemical energy-storage devices, micro-SCs can offer power densities that are much larger than those of conventional batteries and SCs due to the short ion diffusion length. [ 154–156 ] Recently, great effort has been paid to increase the energy and power densities of micro-SCs by using the nanomaterials as the electrode. Various nanostructured mate-rials, such as onion-like carbon, [ 154 ] carbide-derived carbons, [ 157 ] graphene, [ 156,158,159 ] hydrated graphite oxide, [ 155 ] graphene quantum dots, [ 160 ] MnO 2 , [ 161–163 ] PPy, [ 164 ] and PANI, [ 165,166 ] have been utilized as electrode materials for micro-SCs.

Important for fl exible miniaturized energy-storage devices, fl exible micro-SCs on to a chip can be integrated with micro-electronic fl exible devices to work as stand-alone power sources or as effi cient energy-storage units, to which great interest has been paid recently. Commonly, the micro-SCs exist in the form

of interdigital microelectrodes, because of the shortened path lengths for ion diffusion and more effective utilization of the electro-chemical active surface of the electrode mate-rials. Wang et al. designed a fl exible micro-SC microelectrode based on the patterned PANI nanowires array fabricated by micro-fabrication technology and in situ chemical polymerization approach. [ 165 ] By using a photolithography technology, the interdigital-like microelectrode was fabricated on a fl ex-ible PET chip, followed with thermal evapo-rating Au/Cr layer on the pattern as cur-rent collector. PANI nanowires arrays were deposited in situ on the Au layers, and PVA/H 2 SO 4 gel electrolyte was drop-cast onto the active materials to form micro-SCs. The fl exible micro-SC acquired super volumetric capacitance of 588 F cm −3 and good rate capa-bility. Using similar fabrication technology, Si has also been developed for a fl exible micro-SC based on MnO x /Au multilayers. [ 162 ] The as-fabricated micro-SC exhibited a maxi mum energy density of 1.75 mW h cm −3 and a maximum power density of 3.44 W cm −3 , which are both much higher than the values obtained for other solid-state SCs. Furthermore, CV curves measured

without strain and in bending states showed a similar capaci-tive behavior, with a capacitance loss less than 0.09%, demon-strating its high stability. To increase the energy density and power density of the micro-SCs, rGO was selected and used as the active electrode. By combining photolithography with selec-tive electrophoretic techniques, Niu et al. fabricated ultrathin rGO interdigitated microelectrodes on a PET substrate coated with Au fi lm ( Figure 14 a), and micro-SCs were also achieved using PVA/H 3 PO 4 as a gel electrolyte (Figure 14 b). [ 167 ] The spe-cifi c capacitance of the rGO micro-SC was about 285 F g −1 at a scan rate of 5 mV s −1 , much higher than that of the of con-ventional rGO SCs (Figure 14 c). The capacitances normalized to the geometrical area and volume of the rGO micro-SC are 462 µF cm −2 and 359 F cm −3 . The calculated volumetric energy density is 31.9 mW h cm −3 and the maximum volumetric power density is 324 W cm −3 . Signifi cantly, the micro-SC on the PET substrate can be operated under bending without obvious CV performance deviation (Figure 14 d), demonstrating that the rGO-based micro-SCs are quite stable under the bending state, and are suitable for fl exible device applications.

As one of the fl exible SCs, great interest has also been paid to stretchable SCs due to their application in stretchable sys-tems. Li et al. developed a stretchable SC by using SWCNTs coated on PDMS as the electrodes and electrospun polyure-thane as the elastomeric separator. [ 168 ] The specifi c capacitance of the stretchable SC is 50 F g −1 at scan rate of 100 mV s −1 . Interestingly, when a strain of 31.5% was applied, the capaci-tance of the device improved compared with that of the original unstretched state. However, as for the stretchable micro-SCs, it is diffi cult to fabricate interdigital microelectrodes using micro–nano fabrication technologies on stretchable substrates.


Figure 13. a) Rate capability of the solid-state Co 3 O 4 /RuO 2 //Co 9 S 8 -ASC device at different cur-rent densities. b) CV curves collected at the scan rate of 100 mV/s for the solid-state ASC device under normal, bent, and twisted conditions. a,b) Reproduced with permission. [ 142 ] Copyright 2013, American Chemical Society. c) SEM image of the Co 3 O 4 nanowires on a nickel fi ber. d) CV curves of the ASC assembled using Co 3 O 4 nanowires and graphene with the increase of the potential window. c,d) Reproduced with permission. [ 149 ] Copyright 2014, Wiley.

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To achieve mechanical stability on stretchable substrates, Kim et al. developed a 2D planar SC array on an a PDMS substrate. They designed long and narrow serpentine interconnections in a neutral plane and a planar micro-SC array. [ 169 ] The 2D planar SC was fabricated by using patterned SWCNTs as electrodes 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 , and no obvious damage or defects were observed in all the parts of the device, including the SWCNT electrodes and intercon-nections after stretching by 30%, confi rming their potential application in various future fl exible and stretchable electronics.

For the mentioned micro-SCs, the fabrication methods com-monly involve the photolithography technologies or employ masks for the patterns on the substrates, which are awkward and expensive for building cost-effective devices with large area for practical applications. Therefore, developing simple and low-cost technologies that don’t require masks or other com-plex operations while producing high-performance micro-SCs are strongly desired. El-Kady et al. introduced a direct fabrica-tion of micro-SCs based on the interdigitated graphene using a

consumer-grade LightScribe DVD burner. [ 159 ] In their report, GO fi lm was fi rst coated on the disc, followed by converting into gra-phene selectively using the photothermal effect. Thus the patterned-graphene-inter-digitated electrodes with nearly insulating GO fi lm as the separator can be obtained by using the precision of a laser. Unlike conven-tional 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 14 e). These micro-SCs exhibit an ultrahigh power of 200 W cm −3 with high fl exibility and can be bent and twisted without the structural integrity dis-ruption. As shown in Figure 14 f, CV curves of the micro-SC with different bent and twisted states at 1000 mV s −1 showed superior elec-trochemical stability even under bending and twisting conditions, indicating its excellent fl exibility and mechanical stability.

Different from the planner micro-SCs on one-chip, fl exible fi ber-shaped SCs (FSCs), other micro-SCs, showing incom-parable advantages for direct use as fl ex-ible, 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 fi bers, [ 171,172 ] gra-phene, [ 173,174 ] Co 3 O 4 nanowires, [ 149 ] and other composites, [ 175–178 ] have been designed to fabricate fl exible FSCs, exhibiting excel-lent electrochemical performance and high mechanical stability. In 2012, Zou’s group introduced a novel fl exible FSC that con-

sists of two fi ber electrodes, a helical spacer wire, and an elec-trolyte ( Figure 15 a). [ 170 ] In their work, commercial pen ink is employed as the active material for the fi rst time, and remains robust after 15 000 electrochemical cycles. Flexible FSC using Au-coated plastic fi ber as the current collector and substrate and a PVA/H 2 SO 4 gel electrolyte as the separator demonstrated high fl exibility (Figure 15 b). The FSC capacitance dropped only slightly after the high bending, revealing its good mechanical performance. CNT fi bers have been widely used as the active electrodes for FSCs due to their high conductivity and excel-lent mechanical stability. To improve their electrochemical per-formance, many composites based on CNT fi bers have been designed. Choi et al. deposited MnO 2 particles on to CNT bun-dles to form CNT/MnO 2 composite yarn SCs. [ 175 ] The highest volumetric capacitance of about 25.4 F cm −3 was when PVA/KOH was used as the gel electrolyte. Bending cycling tests showed that there was no signifi cant capacitance drop after the 1000th bending at a 90° bending angle (Figure 15 c). In addi-tion, a fl exible composite FCS based on a novel multiwalled MWCNT/ordered mesoporous carbon (OMC) composite fi ber


Figure 14. Optical images of: a) rGO patterns on PET with Au fi lm and b) micro-SC on PET. c,d) The specifi c capacitance (c) and CV curves (d) at different bending states of the rGO micro-SC. a–d) Reproduced with permission. [ 167 ] Copyright 2013, Wiley. e) Photographs of the direct writing of a microdevice. f) CVs collected under different bending and twisting conditions at 1000 mV s −1 . e,f) Reproduced with permission. [ 159 ] Copyright 2013, Macmillan Publishers Limited.

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is developed utilizing MWCNT fi bers for rapid charge separa-tion and transport and high surface area of OMC for high spe-cifi c 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. Interest-ingly, the twisted FSC could be woven into textile structures (Figure 15 d), 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 diversifi ed 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 sig-nifi cant importance for specifi c applications including self-powering systems, micro-electromechanical systems, and portable/wearable personal electronics. [ 179–181 ] In this section, we will briefl y 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 self-charging in integrated devices combining

both energy generation and energy storage may provide a novel method of developing a new mobile power source for both self-powered 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 16 a shows nano-generator-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-har-vesting devices mainly focus on DSSCs. Xu et al. [ 185 ] presented a novel stack-integrated photo-supercapacitor (PSC) thin-fi lm device composed of a DSSC and an SC built on bipolar anodic titanium oxide (ATO) nano-tube arrays. The optimized PSC exhibits an impressive overall photoelectric conversion and storage effi ciency up to 1.64% with a fast response. Moreover, no performance degradation during the 100 photocharge/galvanostatic discharge cycles reveals its excellent cycling capability. Furthermore,

integrated self-charged SCs containing fi ber-shaped DSSCs and fi ber-based SCs have been studied extensively and exhibit


Figure 15. a,b) Schematic diagram (a) and photograph (b) of a fl exible FSC. a,b) Reproduced with permission. [ 170 ] Copyright 2012, Wiley. c) CV plots of solid-state CMY SC before and after a bending test. c) Reproduced with permission. [ 176 ] Copyright 2013, Wiley. d) Microscopy image of an EDLC wire woven into a polyurethane textile. d) Reproduced with permission. [ 177 ] Copy-right 2013, Wiley.

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 fi ber-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) Repro-duced with permission. [ 189 ] Copyright 2014, The Royal Society of Chemistry.

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potential applications in electronic textiles where a wire struc-ture is required. [ 186–188 ] Figure 16 b shows the schematic dia-gram of the fi ber-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 diversifi ed func-tions. Very recently, as demonstrated in Figure 16 c,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 supercapac-itor 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 fl exible LED system integrated with a bendable LIB wrapped with PDMS sheets, which exhibits excellent fl exibility. [ 53 ] Hou et al. also demonstrated fl exible photodetectors based on SnO 2 cloth powered by fl exible LIB and high-performance photo-detectors based on CdSe nanowires integrated with a fl exible in-plane SC. [ 37,78 ] Very recently, Wang et al. developed a fi ber-based, fl exible integrated system to simultaneously realize both fl exible energy storage and fl exible optoelectronic detection on a single fi ber device. [ 149 ] In this integrated system, a fi ber-shaped fl exible ASC, fabricated using titanium wire/Co 3 O 4 nanowires as the positive electrode and graphene coated on carbon fi bers as the negative electrode, was used as the energy-storage and energy-supply device; graphene was also used as the light-sensitive material. By detecting the leakage current under light illumination, a fl exible photodetecting device was obtained. In their work, an all-fl exible 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 signifi -cant importance for specifi c applications including large-area wireless environmental sensing, chemical and biosensing, and in situ medical-therapy monitoring.

5. Conclusions and Future Outlook

In this review, the recent progress in fl exible LIBs and fl exible SCs are summarized systematically. The fabrication of fl exible energy-storage devices requires a fl exible current collector, fl ex-ible electrode materials, a fl exible solid-state electrolyte, and a fl exible encapsulating material. Although great achievements regarding these factors have been obtained by the mentioned fl exible energy-storage devices, many drawbacks still exist and need to be conquered to improve the performance of the fab-ricated fl exible LIBs and SCs. For example, as for fl exible 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 fl exible SCs usually employ a gel electrolyte, which is in a quasi-solid state. To further improve the performance of fl exible energy-storage devices, the fol-lowing research aspects should be considered and more work should be launched to conquer the current drawbacks.

i) Although a series of fl exible energy-storage devices have been fabricated, the energy and power densities still need to be improved. Discovering new electrode materials with a high specifi c 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 fl exible energy-storage de-vices with novel structural designs to meet the different de-mands of fl exible electric devices. A noteworthy technologi-cal advance that has recently received considerable attention is fi ber-shaped LIBs and SCs with extreme omni-directional fl exibility, which frees the cell designer from rigid con-straints. [ 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 fl exible energy-storage devices will be a promising fi eld in the near future. Recently, printing technologies have been booming all over the world because of their large-scale, good fl exibility, and low-lost features. Prospective fl exible displays and lighting, thin-fi lm solar cells, radio frequency identifi cation (RFID) tags, sensors, and so on, can be fab-ricated directly by printing technologies. Therefore, future effort regarding printable energy-storage devices that can provide large-scale, cheap production processes and high fl exibility 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-stor-age devices currently arouse great interest, and many achievements have been obtained, while seldom realizing full fl exibility. Flexible integrated energy-storage systems composed of fl exible power sources and fl exible electronic devices could be a hopeful research direction. We believe that fully fl exible integrated energy-storage systems will emerge as a development promoter that could expedite the advent of the smart, ubiquitous and fl exible integrated sys-tems.

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:

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