Nanostructured graphene-based materials for flexible energy storage

Author’s Accepted Manuscript Nanostructured graphene-based flexible energy storage devices

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Xiaotian Guo, Shasha Zheng, Guangxun Zhang, Xiao Xiao, Xinran Li, Yuxia Xu, Huaiguo Xue, Huan Pang www.elsevier.com/locate/ensm

PII: DOI: Reference:

S2405-8297(17)30249-0 http://dx.doi.org/10.1016/j.ensm.2017.07.006 ENSM181

To appear in: Energy Storage Materials Received date: 19 June 2017 Revised date: 9 July 2017 Accepted date: 11 July 2017 Cite this article as: Xiaotian Guo, Shasha Zheng, Guangxun Zhang, Xiao Xiao, Xinran Li, Yuxia Xu, Huaiguo Xue and Huan Pang, Nanostructured graphenebased materials for flexible energy storage devices, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2017.07.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nanostructured graphene-based materials for flexible energy storage Xiaotian Guo, Shasha Zheng, Guangxun Zhang, Xiao Xiao, Xinran Li, Yuxia Xu, Huaiguo Xue and Huan Pang* School of Chemistry and Chemical Engineering, Yangzhou University Yangzhou, 225009, Jiangsu, P. R. China. [email protected] [email protected] Homepage: http://huanpangchem.wix.com/advanced-material

Abstract Graphene comprising sp2 hybridized carbon atoms has attracted ever-increasing attention for energy storage owing to its two-dimensional cellular structure, which brings about its unique electronic, thermal, mechanical, chemical characteristics and extensive applications. The recent rapid development in energy storage devices with good flexibility has attracted much interest, which will be a pivotal advantage in modern electronics. Graphene-based materials play a significant role in flexible energy storage devices because of their characteristics such as high power density, long cycling life, and short charging time. This review mainly focuses upon flexible supercapacitors and rechargeable batteries (lithium-ion batteries, lithium-sulfur batteries and sodium-ion batteries) based on graphene-based materials. Furthermore, future perspectives and challenges of graphene-based nanomaterials for FESDs are briefly discussed. Graphical abstract

0

Keywords Graphene; flexible; supercapacitor; rechargeable battery

1. Introduction Today, global energy consumption is growing strikingly owing to the increasing energy demand. As a consequence, energy storage devices including supercapacitors (SCs) and rechargeable batteries including lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs) and sodium-ion batteries (SIBs) are widely studied across the globe.[1,2] Moreover, energy storage devices with flexibility have sparked a lot of research efforts, due to their potential application in the next-generation flexible electronic devices.[3–5] Traditional energy storage devices are chiefly based on crisp materials, which are unsuitable for flexible electronics.[6,7] Take conventional LIBs as an example, active electrode materials are mostly assembled with metal current collectors, which use Al foils as positive electrodes and Cu foils as negative electrodes. It is easy for these collectors to be separated from the opposite surface, and it is hard to maintain the original shape when bent.[8,9] An ideal flexible electronic device is required to maintain high energy density and power density when bent, folded or stretched. It should also possess stable electrochemical performances as well as operation safety.[10] However, most of the present-day flexible devices cannot satisfy these demand.[11] Recently, the rapid growth in nanostructured materials has facilitated the investigation of flexible energy storage devices (FESDs) owing to their large specific surface area (SSA), high electrochemical activity and reduced ion transport distance.[12–34] Graphene with two-dimensional (2D) cellular structure discovered by Novoselov and Geim in 1

2004 has gained great research interest. Due to its eminent physical properties such as good mechanical strength (∼1 TPa), high electrical conductivity and high specific surface area (SSA) (2630 m2 g-1) as well as good chemical stability, thermal stability and mechanical flexibility, graphene-based materials have been widely explored as electrode materials for FESDs.[35–38] Take electrochemical double layer capacitors (EDLCs) as an example, the intrinsic electrochemical double layer capacitance of single-layer graphene was measured to be ~21 μFcm-2. Thus, if the entire surface area of graphene could be made full use of fabricating a SC, the SC can deliver a theoretical electrochemical double layer capacitance of ~550 F g-1, which essentially sets the upper limit capacitance for all carbon materials such as CNTs, carbon fiber, activated carbon and graphene. However, the sheet-to-sheet Vander Waals interactions can lead to the restacking of graphene sheets (GSs), to name one problem in its application process. Generally, the effective SSA of graphene related with the number of graphene layers would be obviously decreased and thus make a rapid loss in the transportation of ions during charging/discharging. In order to prevent the restacking of graphene layers, curved and crumpled graphene with morphologies of 1D graphene fibers, 2D graphene films and 3D graphene foams have been developed. Moreover, with regard to its 2D structural properties, graphene has also been used as an ideal building block for controllable functionalization with other electroactive components, such as metal oxides and conducting polymers, and the resulting graphene-based hybrid nanostructures exhibit desirable properties with high power density and energy density for FESDs.[39] Apart from above electrochemical performances, flexible characteristics of graphene materials are also of great importance. According to the structural definition, nanostructured graphene 2

materials are classified into graphene nanoribbons, graphene micro-spheres, balls, nanoscrolls, and their hybrids, which can assemble into 1D graphene fibers, 2D graphene films and 3D graphene foams. Among them, one dimensional (1D) fibers constituted by 2D microscopic rGO sheets stacked in the fiber axial direction, 2D films with alignment of rGO sheets and three dimensional (3D) foams with porous structure usually possess good mechanical strength, which result in the good flexibility of FESDs. As shown in Figure 1,[40–51] the publications of graphene-based materials for FESDs obviously increase in recent two years in order to meet the industrial demands of FESDs and much progress has been witnessed. For example, graphene-based FESDs with light weight can be bent, rolled, twisted and stretched and be assembled into wearable displays and touch-screens, such as popular curved screen mobile phones and LCD TVs with curved screens, which exemplify the growing field of flexible electronics. Due to the sheer number of publications on graphene-based FESDs, it is necessary to sum up advances in this filed systematically, and put forward further trends.[52] While there are a large number of reviews regarding SCs, LIBs and others, there are only a few review articles focusing on graphene-based FESDs. Some excellent reviews are involved in LSBs, LIBs and SCs, respectively.[53–65] For instance, Liang et al. focused on the evolution of the functionality of carbon materials for application in LSBs.[66] Yu et al. reviewed the recent developments on graphene-based LSBs, focusing on the applications in sulfur positive electrodes, lithium negative electrodes, and as interlayers.[67] Yuan et al. mainly discussed the applications of CNTs and graphene in LIBs.[58] With regard to graphene-based materials for application in SCs, Zheng et al. mainly discussed the developments of graphene-based materials for application in asymmetric SCs (a-SCs).[68] The 3

critical assembly principle, standard methods of performance evaluation, major categories of a-SCs, positive and negative electrode materials, and the prime graphene-based materials are reviewed. Wu et al. specially reviewed the recent developments towards designing and controlling the architectures of graphene-based electrodes for SCs.[69] Chen et al. reviewed the developments and synthesis of graphene-based electrodes for SCs.[70] Apart from above, reviews about graphene-based materials for energy storage devices are also comprehensive. Lv et al. reviewed the current uses of graphene-based materials in electrochemical energy storage devices and demonstrated their advances.[71] El-Kady et al. have discussed the current status of graphene in energy storage with specific emphasis on the processing of graphene into electrodes.[56] More importantly, FESDs based on various carbon materials are increasingly popular. Meng et al. focused on the flexible fiber-shaped SCs with special emphasis on the material selection, assembly method, and the electrochemical and mechanical performance.[72] Wen et al. have discussed the recent developments toward CNTs and graphene for FESDs.[10] Akinwande et al. reviewed 2D materials including graphene, MoS2, WSe2, Phosphorene, h-BN flexible nanoelectronics covering progress, prospects and contemporary challenges.[63] Electrodes play an important role on the electrochemical performances and mechanical properties of the FESDs. Therefore, we can classify the electrodes according to graphene and active materials, such as metal oxide, conductive polymers. In this review, the advances in graphene-based FESDs are comprehensively described. Firstly, we review the latest studies of flexible SCs based on pure graphene electrodes, metal oxide/graphene electrodes, conductive polymer/graphene electrodes and so on. The electrochemical properties and flexible characteristics of 1D graphene fibers, 2D 4

graphene films, 3D graphene networks (foams) are discussed in detail. Secondly, flexible graphene-based rechargeable batteries such as LIBs, LSBs, SIBs are given in this review. The electrochemical performances and flexible characteristics of the electrodes or FESDs are reviewed in detail. Pure graphene electrodes, metal oxide/graphene electrodes and other composite electrodes are applied for flexible LIBs. The electrodes of flexible LSBs include sulfur/graphene electrodes, Li2S/graphene electrodes. Besides, there are some graphene-based electrodes employed in flexible SIBs. Finally, some new-type FESDs and practical applications of FESDs are introduced, followed by challenges and perspectives on the future development of FESDs.

Figure 1. Bar chart of graphene-based materials for FESDs in recent years.

2. Flexible supercapacitors

Conventional energy storage devices due to their stiffness, heavy weight and large volume features can hardly be applied in FESDs.[73–75] Accordingly, researchers have made a lot of effort to produce light weight, flexible and portable energy storage devices in recent years.[46,76–80] To some extent, FESDs mainly include stretchable batteries and SCs, micro-scale batteries and 5

micro-SCs,[81–89] novel flexible Li-O2 batteries, Al-ion batteries. However, the development of flexible SCs and batteries becomes more and more comprehensive compared to new Li-O2 batteries, Al-ion batteries in recent years. Among them, SCs (also called as electrochemical capacitors (ECs)), are an important kind of FESDs.[90] Graphene-based materials possess excellent features, such as high SSA, good electrical conductivity, and superior chemical stability.[91] Thus, it has great potential in flexible SC applications. According to the energy storage mechanisms, SCs are divided into EDLCs, pseudo-capacitors and hybrid capacitors.[92] EDLCs store electrical energy, by accumulating electrostatic charges. Factors such as electrical conductivity, pore structures as well as SSA have been the basic elements affecting the electrochemical performances of EDLC electrodes.[65,93,94] Meanwhile, energy storage for pseudo-capacitors is based on reversible and fast redox reactions. Therefore, the values of the capacitance and energy density are basically lower than pseudo-capacitor ones. The electrochemical performances of pseudo-capacitors are strongly affected by such factors as theoretical capacitance and electrical behavior of electrode materials.[95–97] It has been generally considered that electrodes play a critical role for SCs, which is why a large amount of studies have been made in this respect. The key challenge of fabricating high performance SCs with flexibility is to design and synthesize electrode materials with superior mechanical flexibility, high energy density, power density and superb cycle stability, combined with flexible current collectors and compatible electrolytes in a flexible assembly.[72] A brief comparison of the recent investigations on the electrochemical properties and flexible characteristics of flexible graphene-based SCs are shown in 6

Table 1 and Figure 2. The approaches to flexibility of SCs including the microstructures of materials, current collectors and the use of electrolyte are summarized in detail. Microstructures of 1D fibers, 2D films and 3D foams show superb flexible performances, which can be attributed to these microstructures with good mechanical strength. Besides, solid electrolytes including PVA/H3PO4, H2SO4-PVA, PVA/LiCl can protect the FESDs from current leaks and tolerate the mechanical stresses.[10,61] Table 1. Graphene materials for flexible SCs. Pure G

Materials

Approaches to flexibility of devices

Capacity of SCs

RGO RGO

1F, H3PO4/PVA/H2O 1F, PVA/H3PO4/Na2MoO4 1F, H2SO4-PVA

7.5 μF cm-1, 50 mV s-1 38.2 mF cm-2, 0.5 mA

1F, Acetonitrile with 0.1 M NaClO4 2F, PET substrate, H3PO4/PVA 2F, PET substrate, PVA/H3PO4 2F, ITO-PET, Ca(NO3)2-SiO2 3D network, 6 M KOH 3F, 5 M KOH 3F, PET substrate, 6 M KOH 2F, 1 M Na2SO4 2F, PVA/H3PO4 2F, PVA/H3PO4 2F, 3 M KOH 3F, PVA-KOH 3D network, PET substrate, Na2SO4 2F, H2SO4-PVA 2F, Au/PET substrate, H2SO4-PVA 2F, Ti foils, 1 M LiClO4/PC 2F, Au/PET substrate, PVA-LiCl 2F, Rose petal, PDMS, H2SO4 3F, 1 M H2SO4 2F, Pt foils, 1M H2SO4 2F, PVA-H3PO4 1F, H2SO4-PVA 2F, 6 M KOH 1F, PVA-H3PO4 2F, 6 M KOH 2F, carbon fabric, PVA/H3PO4 2F, Au/polyimide film, 1 M KOH 2F, PVA-H3PO4

3.67 mF cm-2, 80 mA cm-2 141.2 F g-1, 1 A g-1 3.67 mF cm-2, 1 A g-1 774 F g-1, 0.1 A g-1 220 F g-1, 2 A g-1 206 F g-1, 0.5 A g-1 100 F g-1, 0.5 A g-1 — 82.6 mF cm-2, 0.06 A cm-3 51.2 F cm-3, 2 mV s-1 — 440 F g-1, 0.45 A g-1 — 175 F g-1, 0.5 A g-1 418 F g-1, 1 A g-1 52.5 mF cm-2, 0.1 A g-1 11 718 μF cm-2, 0.25 A m-2 — 939 F g-1, 1 A g-1 — 140 F g-1, 0.5 A g-1 53.56 mF cm-2, 0.43 mA cm-2 83.2 F g-1, 10 mV s-1 97.73 mF cm-1, 2 mV s-1 255 F g-1, 10 mV s-1 562 F g-1, 0.7 A g-1 112 mF cm-2, 2mV s-1

2F, PET substrate, H2SO4-PVA 3F, Au/chrome-coated polyimide substrate, H2SO4-PVA 2F, PET substrate, H2SO4-PVA 2F, Au/PET substrate, PVA/LiCl 2F, Au/PET substrate, H2SO4-PVA

140.8 F g-1, 1 A g-1

GF@3D-G

MO/G

CP/G

Others

RGO-GO-rGO GO-rGO LSG GSs 3D G RGO 3D G RGO/TiO2 MnO2/G MnO2/rGO Fe2O3/rGO Fe2O3/G G/MnO2 RuO2-IL-CMG RuO2-rGO RGO/V2O5-rGO V2O5.H2O/G PANI/rGO FRGO-F/PANI CCG/PANI-NFs RGO-PPy GO/carbon Pillared GP GHs/MWCNTs-CT RGO-cellulose fibers CF/VAGN/Mn3O4 NiO-GNS/PANI GO/PEDOT-CNTs SRGO NS-HGH CPSC-3rGO VOPO4/G TP/G

1.2 mF cm-2, 50 mV s-1

33.4 mF cm-2, 10 mV s-1

— — 8,360.5 μF cm-2, 0.2 A m-2 3.9 mF cm−2, 1 mV s−1

Stability or cycle life of SCs 90%, 1000 ☆ ~100%, 2500 —, 5000 ☆

Flexibility

Ref [90] [98]

93.4%, 1000 94%, 5000

Bent 0~120o Bent Compressed, stretched Bent Bent Bent 0~180o Bent Bent 120o Bent Bent 0~150o Bent Bent 0~180o Bent 0~180o Bent, twisted Bent 90o Bent 0~180o Bent, twisted Bent 180o Bent Bent Folded, bent Bent Bent Bent 180o Bent 0~90o Bent Bent Bent 0~180o Bent 0~135o Bent 0~90o Folded, twisted, rolled Bent 120o Bent

95%, 500 ☆ 96%, 2000 86%, 10,000 ☆

Bent, twisted Bent Bent 0~60o

[128] [129] [85]

—,160 ☆ —, 2000 ~95%,1000 ☆ ~97%, 10000 >80%, 10000 ☆ 91.5%, 1000 ☆ 95%, 2000 — 96%, 10,000 96.8%, 3000 81.3%,10000 89%,2200 ☆ 92%, 200 ☆ 95%, 2000 ☆ 95.1%, — ☆ >85%, 8000 95%, 2000 85%, 1000 88.7%, 5000 155 F g-1, 800 97%, — ☆ 90.1%, 10 ☆ 95.65%, 2000 90.75%, 500 ☆ 90%, 5000 ~100%, 10,000 86%, 1000 ☆ 97.5%, 5000

[99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [14] [111] [112] [113] [114] [115] [16] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127]

Note: “☆”: Stability or cycle life under deformed states (Ref [112]: bent, twisted; others: bent) MO: Metal oxide; CP: Conductive polymer; Graphene: G; 1F:1D fibers; 2F: 2D films; 3F: 3D foams.

7

Figure 2. The Bar chart of flexible graphene-based SCs including the factors of electrochemical performances and flexible characteristics, the relationships between materials categories and performances (electrochemical performances and flexible characteristics).

2.1. Pure graphene

Although various graphene-based materials including graphene oxide (GO) and reduced graphene oxide (rGO) are used as SC electrodes, pure graphene electrodes are essential to researches on FESDs. On one hand, graphene as one of the carbon materials are EDLC electrode materials. Therefore, the values of their capacitance and energy density are basically higher than these of EDLCs as shown in Table 1. On the other hand, electrochemical performances are related with the effective SSA of graphene. Therefore, in order to reduce the restacking of graphene, curved and crumpled graphene structures including 1D fibers, 2D films and 3D foams have been developed. 1D graphene fibers possess high SSA, superb electrical conductivity, good mechanical strength, excellent flexibility and electrochemical performances. For example, Meng et al. designed a 8

GF@3D-graphene

(GF@3DG-G)

framework.[99]

The

deposited

graphene

layers

were

homogeneously distributed along the entire fiber (Figure 3d-f). The fiber SC kept a steady capacitance of ca. 30-40 µF even when bent over 500 cycles, demonstrating the good stability of fiber SC when deformed (Figure 3g,h). Hu et al. prepared rGO-GO-rGO fibers through region-specific reduction of GO fibers under laser irradiation.[100] The as-fabricated SC based on fibers as electrodes and GO as the separator exhibited good stability after 160 bending tests. Li et al. prepared a fiber-shaped solid SC based on electrochemically rGO, showing superior electrochemical performances and good flexibility.[90] Besides, a flexible cable-type SC (FCSC) based on rGO nanosheets was fabricated by Veerasubramani et al. and indicated improved electrochemical performances using redox additive electrolyte.[98] Moreover, the device also showed good flexibility when bent. The 2D freestanding graphene films structure endow the graphene electrodes with robust mechanical flexibility, which can be directly used as self-standing or substrate-supporting electrodes for SCs as shown in Table 1.[130–133] El-Kady et al. prepared graphene directly from GO films via laser reduction of a LightScribe DVD optical drive (Figure 3a).[102] Compared with GO, the obtained laser-scribed graphene (LSG) films exhibited an improved electrochemical behavior with a nearly rectangular cyclic voltammetry (CV) shape at 1000 mV s-1. Furthermore, a lightweight FESD based on LSG films was fabricated and showed good stability (over 4 months of testing) as well as superb cycling stability (>97%, after 10,000 cycling tests) (Figure 3b). Meanwhile, Figure 3c illustrates that there was no obvious change in CVs when the device was bent from 0 to 180°and there was just ~5% decay after more than 1000 cycles under bending, which demonstrated good 9

flexibility and stability. Similarly, Jung et al. developed an innovative approach to use direct printing and photo-thermal reduction of GO to fabricate a highly porous pattern of interdigitated SC.[134] The printed-rGO-based SC was bent at different angles and the performance barely varied. Xue et al. have successfully prepared multiscale, planar patterning of GO-rGO by specific reduction of GO under a facile irradiation.[101] The assembled SCs based on the GO-rGO composite electrodes exhibited its capacitance of 141.2 F g-1. Ramadoss et al. prepared a 3D graphene/graphite paper via a facile chemical vapor deposition (CVD) approach.[135] The assembled SC delivered a maximum capacitance of 260 F g-1 in a three-electrode system and 80 F g-1 in a full cell as well as a high capacitance retention. Moreover, the SC maintained good electrochemical performances even under deformed states (bent, rolled, or twisted), demonstrating its good flexibility of the assembled device. Different from above 2D films-based SCs, Zheng et al. adopted a novel printable strategy to fabricate arbitrary-shaped SC using electrochemically exfoliated graphene as both anode and cathode, GO as a separator on a PET substrate.[133] The as-fabricated SC delivered a volumetric capacitance of ∼280 F cm−3 by using a redox electrolyte and exhibited no obvious decay for rectangle-shaped SCs after 10,000 cycling tests, demonstrating superb flexibility of the arbitrary-shaped, graphene-based planar sandwich SC.

Figure 3. a) Schematic diagram of the fabrication of LSG-SCs; b) A shelf-life test showing good 10

stability for over 4 months; c) CVs when the LSG-SCs were bent at 1000 mV s-1. Reproduced with permission. [102] Copyright 2012, American Association for the Advancement of Science. d) A SEM image of a GF@3D-G; e) An enlarged view of d); f) The edge view of a GF@3D-G; g,h) The capacitance stability of GF@3D-G fiber SC when bent and straight, respectively. Reproduced with permission.[99] Copyright 2013, Wiley-VCH. i) Schematic illustration for synthesis of the rGO hydrogel film and the fabrication of a SC; j) A photo of the belt-like flexible SC; k) Capacitance retention after 1000 bending tests. Reproduced with permission.[105] Copyright 2015, The Royal Society of Chemistry.

Compared with 1D graphene fibers and 2D graphene films, 3D graphene foams with ordered porous structure and high flexibility possess large effective SSA, fast electron transfer and ion transfer, which have been widely studied for SC applications.[136,137] Generally, graphene foams can be synthesized via thermal and chemical reduction from GO, or directly obtained via CVD method on template. A non-stacked rGO hydrogel film was prepared via an in situ electrochemical reduction by Feng et al. (Figure 3i).[105] The as-obtained films with porous structure possessed a large SSA and could be directly applied as electrodes for SCs without drying. The specific gravimetric and volume capacity of electrochemically rGO film (ERGO) achieved 206 F g-1 and 231 F m-3, respectively. Furthermore, a belt-like packed flexible capacitor based on two ERGO electrodes using titanium mesh as a current collector was fabricated, as shown in Figure 3j. 91.5% of capacitance retention was obtained when bent over 1000 times, which indicated good flexibility of the device (Figure 3k). Apart from these, the capacitor device is capable of powering a light-emitting diode (LED) lamp for over two minutes. Chemically rGO also called chemically converted graphene (CCG), can be well dispersed in water and can self-assemble into oriented 3D graphene hydrogels. Yang et al. fabricated a SC with a high capacitance and maximum energy density of 59.9 Wh L-1 based on the hydrogels by taking advantages of CCG with corrugated 2D structures and 11

self-assembly property.[138] Different from above, Yu et al. fabricated a flexible SC based on CCG as the active material and stainless steel fabrics (SSFs) as the current collector.[139] As a result, the assembled SC delivered a high capacitance of 180.4 mF cm-2 along with a 96.8% capacitance retention after 7500 cycling tests. Besides, the flexibility of the SC was tested and it still showed a capacitance of 96.4% after 800 bending tests. Shi et al. successfully prepared ordered 3D graphene through facile hydrothermal reduction method, which possessed a superior gravimetric capacitance of 220 F g-1 at 2 A g-1 and a superb cycle stability.[104] The capacitance retention maintained >80% over 10,000 cycling tests even when bent at 200 mV s-1 scan rate.

2.2. Metal oxide/graphene

So far, a variety of pseudo-capacitor electrode materials including metal oxides and conducting polymers (CPs) have been developed.[140] According to the electrical behavior and charge storage mechanism in SCs, EDLC electrode materials generally have a higher rate capability, higher power density, longer operation life, lower capacitance and energy density than pseudo-capacitor ones.[141] Thus, it is worth considering that combining EDLC materials with pseudo-capacitive materials may combine their respective advantages.[142,143] Metal oxides such as MnO2,[108,109,144–146] Fe2O3,[14,110,147] Fe3O4,[18] RuO2,[112] V2O5,[114,115] Bi2O3[148,149] nanoparticles (NPs) are the most widely explored pseudo-capacitor materials. Apart from electrochemical performances, microstructures including 1D hybrid fibers, 2D films, 3D foams also play an important role in flexible characteristics of FESDs. A facile wet-spinning method is usually used to fabricate 1D metal oxides/graphene hybrid fibers, such as MnO2 nanowire (nanorods)/graphene hybrid fibers and MoO3 nanorods/rGO hybrid 12

fibers. For example, Ma et al. prepared a MnO2 nanowire/graphene hybrid fibers with a hierarchically structure via a facile wet-spinning process.[108] The broad characteristic peaks in the XRD patterns showed the poor crystallinity of the α-MnO2, which were more favorable for SCs. The SEM images indicated the as-obtained α-MnO2 nanowire structure with 5-20 nm in diameter and 5-10 μm in length and a 3D interconnected porous network structure of crumpled GSs from the cross-section of the fiber (Figure 4a-c). More importantly, a high performance FSSC based on hybrid fibers was assembled and delivered a high volumetric capacitance of 66.1 F cm -3 and a 96% capacitance retention after 10,000 cycles. Besides, high energy density of 5.8 mWh cm -3 and power density of 0.51 W cm-3 were obtained (Figure 4d). With regard to the flexibility of the FSSC, the CV curves under various bending angles and GCD curves after different bending times were tested and showed no distinct difference (Figure 4h,i). Furthermore, the device connected in series can light up a red LED even under knotting state, suggesting its potential in FESDs. Interconnected rGO sheets in the hybrid fibers can serve as mechanical support and electron transport channel for the fibers and thus bring about excellent electrical and mechanical properties. Similarly, Ma et al. also assembled a flexible a-SC using MnO2 nanorods/rGO hybrid fibers as the positive electrode, MoO3/rGO as the negative electrode and H3PO4/PVA as electrolyte.[109] The optimized a-SC delivered a good volumetric energy density of 18.2 mWh cm-3 at a power density of 76.4 mW cm-3. Besides, the a-SC possessed outstanding cyclability, flexibility and stable mechanical properties. Different from above MnO2/rGO, Guo et al. adopted polyester as flexible substrates, wet-spinning method of synthesizing GO/polyester composites, NaBH4 solution immersion of obtaining graphene/polyester hybrid fibers, hydrothermal process for MnO2/graphene/polyester.[145] The effects of MnO2 morphology, 13

electrode structure, mechanical bending and stretching on the electrochemical performance of MnO2/graphene/polyester

composite

electrode

materials

were

studied

in

detail.

The

MnO2/graphene/polyester composite fabrics electrode delivered a maximum specific capacitance of 332 F g-1 at 2 mV s-1 and a FSSC based on the composite fabrics exhibited a specific capacitance of 265.8 F g-1 at 2 mV s-1. Besides, the FSSC also demonstrated stable electrochemical performances under mechanical bending and stretching conditions.

Figure 4. a) SEM image of the MnO2 nanowires; b,c) Cross-sectional SEM images of the MnO2/GO fibers; d) Energy and power densities of the SC compared with commercially available energy storage systems; e) Schematic illustration of the fabrication of the flexible solid state holey rGO-RuO2 SC; f) SEM image of freeze-dried holey rGO-RuO2; g) TEM images of holey rGO-RuO2 (yellow and red circles represents RuO2 NPs and the in-plane nanopores, respectively) h) CV curves under bending at with different angles; i) GCD curves before and after bending for 100, 500 and 1000 cycles. a-d,h,i) Reproduced with permission.[108] Copyright 2015, Elsevier B.V.. e-g) Reproduced with permission.[113] Copyright 2016, Elsevier Ltd.

One fabrication technique of 2D metal oxide/graphene films usually covers three parts: 1) 14

synthesis of GO via a modified Hummers method; 2) mixture of metal oxides with GO via simple solution mixing, in situ growth, electrodeposition, self-assembly or CVD growth; and 3) reduction of metal oxides-GO into metal oxides-rGO. Besides, the reduction process of GO into rGO and subsequent mixture of metal oxides with rGO are also another important fabrication technique. Along with microstructure of 2D films, flexible substrates and electrolyte are also advantageous for FESDs. Amir et al. successfully fabricated porous rGO-RuO2 composites via simple sol-gel and electrophoretic deposition methods (Figure 4e).[113] The holey rGO-RuO2 composites indicated large holey rGO sheets with diameters up to a few tens of microns (Figure 4f) and were well decorated with ultra-small RuO2 NPs with diameters of 1.0-2.0 nm. Moreover, a large number of in-plane pores with sizes of 1.0-4.0 nm could be observed across the whole basal plan of composites (Figure 4g). The assembled flexible SC based on rGO-RuO2 films, Au-coated PET substrates and PVA-H2SO4 gel electrolyte exhibited superior capacitance of 418 F g-1 along with a capacitance retention of 88.5% over 10,000 cycling tests. Moreover, good mechanical flexibility with only 4.9% decrease was achieved when bent 180°. The superior performances of flexible composite electrode can be ascribed to the addition of pseudo capacitive RuO2 NPs, the novel characteristics of rGO with nanopores, and the layered framework of rGO-RuO2 sheets. Choi et al. successfully prepared RuO2-liquid functionalized-chemically modified graphene (IL-CMG) films via a simple method as a positive electrode for SCs (Figure 5a).[112] The RuO2 NPs were uniformly distributed along the IL-CMG films surface (Figure 5c). The optimized asymmetric SC (a-SC) based on RuO2-IL-CMG films delivered specific capacitances of 175, 166, 144, 141, and 139 F g-1 at 0.5, 1, 2, 5, and 10 A g-1, respectively. Remarkably, this device achieved 95% of its initial specific capacitance after 2000 15

cycling tests when deformed in PVA-H2SO4 electrolyte (Figure 5b). These excellent performances show the great potential of RuO2-IL-CMG for FESD applications, which are ascribed to the good mechanical robustness, interfacial contact and electron transfer among the components. 2D V2O5.H2O/graphene hybrid films are ordinarily prepared by a modified Hummers method, solution mixing and subsequent hydrothermal reduction.[114,115] Generally, the V2O5 NPs acted as spacers prevented the stacking of GSs and provide pathways for fast ionic transport and maximize SSA for the EDLCs. Foo et al. successfully assembled a flexible a-SC based on rGO/V2O5-rGO, which exhibited good cyclability, areal power and energy density in both flat and bent state.[114] When tested in flat state, a maximum energy density of 13.3 W h kg density decreasing to 12.5 W kg 13.6 W h kg

−1

−1

−1

was obtained with power

. In bent state, the a-SC delivered a maximum energy density of

. Besides, a flexible V2O5.H2O/graphene based all-solid-state ultrathin-film SC was

successfully prepared by Bao et al. (Figure 5e) and displayed high areal capacitance of 11 718 μF cm-2 at 0.25 A m-2 along with superior cycle life.[115] The as-obtained FSSC also delivered a good energy density of 1.13 mW h cm-2 at a power density of 10.0 mWcm-2. Moreover, the flexibility of the as-fabricated SC was measured via GCD curves and cycling stability tests in a bent configuration (Figure 5f). The capacitance of the SC indicated no obvious decay after 500 cycles, which can be attributed to the ultrathin nature of the 2D thin films. Besides, Liu et al. fabricated oxygen-deficient Bi2O3 (r-Bi2O3)/graphene (GN) via simple solvothermal and solution reduction approaches (Figure 5i).[148] The Bi2O3/GN prepared via solvothermal method and r-Bi2O3/GN synthesized via subsequent reduction treatment demonstrated different morphologies of their architectures as shown in the TEM images (Figure 5j,k). Before reduction treatment, ultrathin Bi2O3 nanosheets can be 16

observed on GN sheets, demonstrating successful attachment of Bi2O3 and GN. After reduction treatment, the r-Bi2O3/GN indicated more close connection with GN sheets compared to Bi2O3/GN, and less standing Bi2O3 nanosheets can be observed, which ensured superb charge transfer through the highly conductive GN. Moreover, the bacterial cellulose (BC) as flexible substrates for r-Bi2O3/GN showed good flexibility and tensile strength of 55.1 MPa, which played an important role in flexible SCs. The flexible a-SC based on r-Bi2O3/GN/BC and Co3O4/GN/BC paper as the negative and positive electrodes delivered a high energy density of 0.449 mWh cm−2, a maximum power density of 40 mW cm−2, and superb flexibility with no distinct capacitance decay under different bending conditions (Figure 5g). Apart from 1D fibers and 2D films, 3D hybrid networks are also another important consideration, which can effectively separate GSs with each other, retain high SSA and thus brings about superior electrochemical and flexible performances, which attribute to the structural morphology of the electrode materials and the influence of the electrode components on the mechanical flexibility of the as-fabricated devices.[150] 3D hybrid networks can be prepared through vacuum filtration, subsequent thermal reduction or electrochemical reduction. He et al. have successfully fabricated a self-standing flexible 3D graphene/MnO2 composite network as an electrode for application in SCs.[111] The MnO2 content with regard to the whole electrode was optimized and the whole electrode reached a maximum specific capacitance of 130 F g-1. Besides, the electrochemical performances and flexible properties of the as-fabricated SC based on 3D graphene/MnO2 composite network, PET membrane and polymer separator were studied. The SC with light weight of <10 mg and thin thinness of ∼0.8 mm achieved a capacitance of 24.2 F g-1 after 17

5000 cycles along with a capacitance retention of 82%. There was no obvious change in the CV curves even when bent the SC and the specific capacitance maintained 92% even after 200 bending tests with a bending angle of 90o, indicating the superb flexibility of the graphene/MnO2-based SC. Hu et al. prepared a hybrid Fe2O3/rGO paper, which possessed higher capacitance of 178.3 F cm−3, compared to pristine rGO paper of 106.2 F cm−3 at 1 mV s−1 in CV test.[110] Liu et al. prepared a well-organized Fe3O4/GS composite paper via a novel three-step method.[151] The Fe3O4 NPs were ~5 nm in size and were grown in situ and uniformly immobilized on GSs. The 3D paper offers superior electrical conductivity and enhances the dispersion of Fe3O4 NPs, further improving the capacity. The Fe3O4/GS paper possessed a specific capacitance of 368 F g-1 and 225 F g-1 at 1 A g-1 and at 5 A g-1, respectively and achieved 245 F g-1 at 5 A g-1 over 1000 cycles, demonstrating a promising potential for FESDs.

Figure 5. a) Schematic illustrations for the preparation of IL-CMG and RuO2-IL-CMG composites; b) Cycling stability of a-SC based on RuO2-IL-CMG composites under different states at 1 A g-1; c) High resolution TEM image of RuO2-IL-CMG hybrids (inset is histogram of the size distribution of RuO2 from TEM images); d) Digital photographs of a flexible cell; e) Schematic illustration of the flexible V2O5.H2O/graphene-based SC; f) Cycling test of the V2O5.H2O/graphene-based SC over 500 cycles; g) CV curves under bend and flat condition at 40 mV s-1, inset shows the device; h) The digital photographs of novel fold type and LED light linked SCs; i) Schematic illustration of the 18

synthesis route toward flexible r-Bi2O3/GN/BC electrode; j,k) TEM images of the Bi2O3/GN and the r-Bi2O3/GN l) Schematic diagram of the cell package; m) Biotemplate replication from a rose flower. a-c) Reproduced with permission.[112] Copyright 2012, The Royal Society of Chemistry. e,f) Reproduced with permission.[115] Copyright 2014, The Royal Society of Chemistry. i-k,g) Reproduced with permission.[148] Copyright 2017, Wiley-VCH. d,h,l,m) Reproduced with permission.[16] Copyright 2016, The Royal Society of Chemistry.

2.3 Conductive polymer/graphene

Apart from above electric active materials, CPs have many advantages, such as low cost, wide voltage window, eco-friendliness, good reversibility and adjustable electrochemical activity. Hence, ever-increasing efforts have been sparked on combining graphene with CPs in order to combine the advantages and fulfill the requirements of high energy density, power density and cyclability. Graphene composites with CPs including polyaniline (PANI),[152–155] polypyrrole (PPY)[118,156] and poly(3,4-ethylenedioxythiophene) (PEDOT)[157,158] have been studied widely for the application in SCs. Among them, PANI is regarded as a promising material due to good capacitive properties, low cost, and simple preparation. Wu et al. fabricated a novel kind of flexible SCs based on paper-like films consisting of CCG and PANI nanofibers (PANI-NFs).[117] A high quality and flexible film named G-PNF30 with 30% CCG weight was obtained. PANI-NFs were homogeneously distributed between CCG layers in G-PNF30 films. The symmetric SC device using composite films showed good stability, a capacitance of 155 F g-1 was achieved over 800 cycles at 3 A g-1. Cong et al. prepared a self-standing graphene/PANI electrode, which exhibited superior flexibility, low weight (0.2 g cm-3) as well as good electrical conductivity (15 Ω sq-1).[159] The graphene/PANI composite paper delivered a higher specific capacitance of 763 F g-1 than pure PANI film of 520 F g-1 at 1 A g-1. 19

Moreover, the composite electrode exhibited a superior cycling stability with 82% of capacitive retention over 1000 cycles, much higher than 51.9 % of PANI film. Yu et al. prepared a novel flexible rGO foam/PANI (FRGO-F/PANI) composite by a simple in situ polymerization method with a high specific capacitance of 939 F g−1.[116] The assembled symmetric SC based on the composite electrodes demonstrated a good cyclic stability with a capacitance retention of 88.7% over 5000 cycling tests. Moreover, CV curves of the SC indicated that there was almost no change when bent 180o, demonstrating its good flexibility. Chang et al. prepared a PANI/rGO composite with a hierarchical multiscale rose flower-based structure via in situ oxidative polymerization (Figure 5m).[16] The as-fabricated SC based on the composite electrode, GP as a current collector and H2SO4 as electrolyte (Figure 5l) achieved a high capacitance of 626 F g-1. The SC displayed good flexibility under different bending deformations (Figure 5d) and showed no distinct change in CV curves. Furthermore, the SC with a five-cell series lit up the red LED with a potential window of 2.2 V even when sharply folded within a small space (Figure 5h). PPy is also a low-cost and easy preparation material, which mostly used one-step electropolymerization method for direct growth of the CPs on the current collector. For example, Zhou et al. prepared PPy/GO composites through electrochemical deposition method.[156] The obtained PPy/GO composites with a fuzzy sheet-like morphology displayed good capacitive performances. The composite possessed a high areal capacitance of 152 mF cm-2 at 10 mV s-1. Shu et al. prepared a novel free-standing rGO-PPy composite paper via electrochemical polymerization, which possessed a uniform layer-by-layer structure with PPy embedded in graphene layers.[118] The hybrid rGO-PPy60 papers with the deposition time of 60 min indicated a greatly improved areal 20

capacitance of 440 mF cm-2 at 0.5 A g-1 and delivered a maximum energy density of 61.3 μWh cm-2 at a power density of 1.2 mW cm-2 Moreover, the capacitive behavior at different bent states was almost the same as that at the original state, suggesting the stability of this FESD. Yang et al. have developed a simple method to fabricate a flexible all-solid-state SC (FSSC) based on rGO/PPy NP composite papers with a layered structure.[29] There was no distinct capacitance decay when bending the SC and a maximum areal energy density of 132.5 Wh cm-2 was obtained, which showed a great potential in FESDs. esides

ehtim i et al. successfully prepared PEDOT/rGO composite

films on flexible PET substrates via a simple electropolymerization and reduction method, which has drawn great attention towards application in FESDs.[157]

2.4 Others

2.4.1 Carbon/graphene

Carbon materials, including carbon nanospheres,[119] CNTs,[83,121,160–162] carbon black (CB),[120] carbon fibers (CFs),[123] graphene and etc., are representative EDLC electrode materials. Thus, EDLC electrode materials generally possess a lower capacitance and energy density than pseudo-capacitor

electrode

materials

including

metal

oxide/graphene

and

conductive

polymers/graphene composites when combing graphene and other carbon materials as shown in Table 1.[122] Apart from the electrochemical performances, many hybrid fiber-shaped SCs with superb flexible properties were fabricated for application in flexible devices owing to its inherent properties of fibers. Zhang et al. have fabricated a flexible fiber-shaped SC electrode with layered structures via a 21

one-step electrophoretic method.[119] The carbon nanospheres were well dispersed and the carbon nanospheres were embedded in GO nanosheets. The fibrous electrode delivered an optimal capacitance of 53.56 mF cm-2 and a superior reversible capacity without an obvious decay of 91.2% over 4000 cycles. Interestingly, the fiber-shaped SC still exhibited its stability even under the bending angle of 90°, which demonstrated its good flexibility. Zhou et al. fabricated a fiber-shaped SC

based

on

a

highly

flexible

graphene

hydrogels/multi-walled

CNTs-cotton

thread

(GHs/MWCNTs-CT) fiber-shaped electrode.[121] The fiber-shaped SC possessed a superior capacitive behavior of 97.73 μF cm-1 at 2 mV s-1 as well as a 95.51% capacitance retention after 8000 cycles. Moreover, the capacitance of the fiber-shaped SC showed no obvious decay under different bending states. By contrast, it even indicated a capacitance improvement of 7.4% at a bending angle of ~180°. The flexible characteristics are highly dependent on its structure of fibers. Besides, Tang et al. successfully fabricated free-standing rGO-wrapped Fe-doped MnO2 composite (G-MFO) and rGO-wrapped hierarchical porous carbon microspheres composite (G-HPC).[163] The obtained G-MFO films as positive electrodes and G-HPC films as negative electrodes were assembled into a-SC devices, respectively. The ASC device exhibited a superior electrochemical stability with 91.6% capacitive retention over 8000 cycling tests even when bent. Moreover, the ASC device showed almost no change in CV curves though when deformed, indicating its superb mechanical stability for application in SCs.

2.4.2 Metal oxide/graphene/carbon

Compared with graphene-based ternary hybrids, binary hybrid materials with metal oxides, CPs or carbon materials also have many defects, including poor cycling behavior, small SSAs, and 22

nonelastic deformation of the transition metal oxides (TMOs) during charge-discharge processes. Hence, the ternary composites of graphene materials may result in excellent electrochemical performances and good flexibility.[164] Co3O4 and Mn3O4 are regarded as ideal materials owing to their low cost and good capacity. Liao et al. prepared a carbon fiber (CF)/vertically aligned graphene nanosheets (VAGN)/Mn3O4 electrode through a hydrothermal method.[123] The surface of the VAGN was all covered by Mn3O4 NPs with a grain size ~10 nm and both sides of the VAGN are anchored with the Mn3O4 NPs. Additionally, a FSSC was assembled by two pieces of the electrode, exhibiting a capacitance of 562 F g-1, high energy density of 50 W h kg-1, high power density of 64 kW kg-1 and superb stability after 10 000 cycles. Moreover, superb cycling stability with only slight decay at the bending angle of 150°, indicated good flexibility of the FSSC, which can be ascribed to the flexibility of the carbon fabric, hybrid fiber structures, PVA/H3PO4 gel electrolyte. Yuan et al. fabricated a flexible and free-standing Co3O4/rGO/CNTs paper electrode for application in SCs.[165] It was found that this novel flexible paper electrode possessed a capacitance of 378 F g-1 and 297 F g-1 at 2 A g-1 and 8 A g-1, respectively. Besides, the flexible paper also showed stable cycling behavior at different current densities. A capacitance retention of 96% was achieved at 2 A g-1 over 3000 cycles without distinct decay. The ideal electrochemical behaviors of the flexible paper electrode were attributed to the hierarchical microstructure.

2.4.3 Metal oxide/conductive polymer/graphene

CPs tend to contract and inflat in volume during the charge-discharge process and therefore brings about poor cyclability. Hence, the preparations of CPs-metal oxide composites have been studied to 23

improve electrochemical properties. Wu et al. fabricated a novel NiO coated graphene nanosheets/PANI (NiO-GNS/PANI) composite film.[124] The as-obtained film delivered a specific capacitance of 1409 F g-1 at 1.0 A g-1 and superior cycling stability with a capacitance retention of 92 % after 2500 cycling tests. Moreover, it also showed good electrochemical stability at different angles as well as long cycling life of 86% capacitance retention over 1000 bending tests. Sankar et al. synthesized CoFe2O4/rGO/PANI (CCGP) and β-Co(OH)2 composite.[166] The as-fabricated SC based on CCGP as negative electrodes and β-Co(OH)2 as positive electrodes delivered a high length capacitance of 9 mF cm-1 at 1 mA and superior cycling performance. Moreover, there was almost no change in CV tests between flat and bent states, demonstrating its flexibility of the cell. Furthermore, in order to demonstrate its practical application, a red LED was powered and the words inside the LED could be clearly seen.

Lee et al. constructed a carbonized PPy-coated SnO2/Co3O4

nanofiber-decorated 3D rGO (CPSC-3rGO) nanostructure.[128] The CPSC-3rGO showed a higher specific capacitance (446 F g-1) than other intermediated materials (150 F g-1 for 3D rGO). Moreover, the 3D graphene-based structure demonstrated good mechanical stability and the resulting SCs possessed a large specific capacitance even under bending and twisting states.

2.4.4 Doped graphene

Substituting for graphene with heteroatoms in the graphitic lattice is an effective strategy of making changes in the original structure and morphology.[167,168] 2D hybrid films and 3D networks endow the SCs based on the composites with good flexibility. Byun et al. developed a facile solution-based method to prepare boron nitride (h-BN)/graphene hybrid film.[15] The as-fabricated SC based on the hybrid film electrodes, Ni current collector and PVA-KOH electrolyte possessed good 24

electrochemical properties as well as flexibility. The flexible SC displayed a volumetric capacitance of ~95 F cm-3 at 50 mV s-1 and kept stable SC operation even after bending. 3D networks mainly

include foams (hydrogel, aerogel), 3D porous frameworks and sponges. Tran et al. prepared 3D nitrogen (N) and sulfur (S) co-doped hole defect graphene hydrogel (NS-HGH) electrodes with a specific capacitance of 536 F g-1.[127] The novel design of the NS-HGH-based FSSC brings about superb electrochemical performance and mechanical flexibility. The SC showed superb cyclability of 94% capacitance retention over 5000 cycling tests at 10 mV s-1. At the same time, Wang et al. fabricated a hierarchically porous hybrid electrode of strongly coupled N-doped porous carbons and graphene aerogel (NPC-GA) with a high specific capacitance of 608 F g-1 in 1 M H2SO4 at 0.1 A g-1.[169] Besides, a flexible SC based on NPC-GA delivered a superior energy density (12.4 Wh Kg-1) and power density (2432 W Kg-1) as well as good cyclability of 92% specific capacitance retention over 10,000 cycles.

2.4.5 Others

Zhou et al. fabricated carbon nanotubes films (CNFs)-loaded GO/PEDOT-CNTs via a electrodeposition process.[125] The assembled device based on CNFs-GO/PEDOT-CNTs delivered a high areal capacitance of 33.4 mF cm-2 at 0.042 A cm-3 and superb cyclability of 97.5% capacitance retention at the 5000th cycle. A binder-free Ag/Co3O4/3D graphene (3DG) composite electrode was prepared by Wang et al. (Figure 6a) and possessed good pseudo-capacitive and cycling performances for both the three- and the two-electrode systems.[170] The flexible Ag/Co3O4/3DG based a-SC delivered a high energy density of 26.7 Wh kg-1 and could even power a red LED for 30min (Figure 6b,c). An ultrathin-film pseudo-capacitor based on VOPO4/graphene composite films 25

was fabricated by Wu et al.[129] as shown in Figure 6e and demonstrated good conductivity and electrochemical properties. Figure 6d demonstrated the synthesis of VOPO4 ultrathin nanosheets, with a thickness of less than six atomic layers. The as-fabricated SC obtained a high redox potential of 1.0 V and areal capacitance of 8 360.5 μF cm-2, resulting in a high energy density of 1.7 mW h cm-2 and a power density of 5.2 mW cm-2 (Figure 6h). Moreover, flexible tests of the SC showed no distinct decay of the specific capacitance over 400 bends (Figure 6f,g). These excellent performances show a great potential in FESDs including portable devices. Wu et al. successfully fabricated all-solid-state pseudo-capacitors based on the 2D heterostructure film of ultrathin Thiophene and electrochemically exfoliated graphene nanosheets (TP/EG).[85] The flexible all-solid-state pseudo-capacitors (ASSSs) were fabricated by two electrodes based on heterostructure films using Au-coated PET substrates and PVA/H2SO4 as both electrolyte and separator, showing a higher areal capacitance of 3.9 mF cm-2 than exfoliated graphene-based SCs of 1.5 mF cm-2 at 1 mV s-1. Additionally, the flexible characteristics of the pesudo-capacitors were also measured in CV curves and cycling tests, indicating good stability even when bent 180 o. These excellent performances of SCs can be attributed to the layered film structure with an integrated synergetic effect of ultrathin Thiophene nanosheets and exfoliated graphene nanosheets.

Figure 6. a) Schematic illustration of preparation process for the Ag/Co 3O4/3DG hybrid; b) The 26

schematic illustration of the flexible SCs; c) Pictures indicating that the flexible Ag/Co3O4/3DG-based SCs can lit up red LED indicators. d) Schematic illustration for 2-propanol-assisted exfoliation process from bulk VOPO4.2H2O to graphene-like VOPO4 nanosheets; e) Schematic illustration of an as-obtained flexible VOPO4/graphene hybrid film-based pseudo-capacitor; f) Cycling test of the VOPO4/graphene hybrid film-based SC over 2000 cycles; g) Cyclability of the VOPO4/graphene-based SC under repeated bending/extending deformed states; h) Performance comparison of various materials. a-c) Reproduced with permission.[170] Copyright 2016, Elsevier Ltd. d-h) Reproduced with permission.[129] Copyright 2013, Macmillan Publishers Limited.

3. Flexible rechargeable batteries

There are dramatic differences between SCs and batteries in power and energy densities. Compared with SCs, batteries possess higher energy density but they have lower power density owing to the low ion flow in redox reactions.[171,172] Hence, rate performance and cycle life of batteries as well as safety operation need more improvements for FESDs. Due to an increasing demand for an electrical energy storage system that is characterized by being small in size, light in weight with high capacity, and high safety, researchers have been trying to explore new electrode materials as well as batteries that are rechargeable, of which LIBs, LSBs as well as SIBs have been brought to the forefront.[173] The advantages and disadvantages of LIBs, LSBs and SIBs are as shown in Table 2, demonstrating a good potential in energy storage systems.

27

Table 2. The advantages and disadvantages of rechargeable batteries. Graphene

LIBs

SIBs

LSBs

Advantages

High energy density;

High abundance of 2.75%;

High abundance;

Good stability;

Low cost of sodium;

Low cost of sulfur;

Good stability;

High theoretical capacity;

weight of 2.3 g mol , low

Good rate performance.

(Anode: 3860 mA h g ;

potential (-3.04 V))

(Standard potential (-2.70 V))

cathode: 1673 mA h g )

(Light density, small atomic -1

-1

-1

-1

High energy density (2600 Wh kg ). Disadvantages

High cost;

Low coulombic effciency.

Low abundance of 0.0065%;

(Larger Na size compared to -1

Low capacity. (Theoretical

Low electrical conductivity;

+

Li, atomic weight of 6.9 g mol ) capacity:

372

mA h g of LiC6; ~ 780 mA h g of Li2C6; ~ 1116 mA h g

Low rate performance. (Large volume changes of sulfur,

-1

-1

Poor stability;

soluble polysulfide)

-1

of LiC2)

Furthermore, a brief comparison of investigations on the applications in flexible rechargeable batteries based on graphene-based materials are illustrated in Table 3. The electrode materials, different morphologies, current collectors, separators along with electrochemical performances and flexible characteristics are summarized in detail. Besides, the electrolytes summarized below the Table 3 also have an impact on the electrochemical and flexible performances of batteries. Table 3. Graphene materials for flexible rechargeable batteries Materials

LIBs

LSBs

SIBs

GNP/GO mTiO2-GS/G MoO3/graphene LTO-FSG CGN/SnO2 Graphene/LiFePO4 MCG Ge-QD@NG/NGF MoS2/graphene Graphene/S GPC-S S-G@PP 3DGS MGP-S S-PDMS/graphene 3DLG MoS2/rGO SnS2@graphene S-doped graphene Sb/rGO NVP@C@rGO

Morp hologi es

Electrodes

Current collector/Separator

2F 2F 2F 2F 3N 3N 3F 3F 2F 2F 3N 3N 3F 3N 3F 3F 2F 1F 2F 2F 2F

Anode Anode Cathode Anode Anode cathode Anode Anode Anode Anode Cathode Cathode Cathode Cathode Cathode Cathode Cathode Anode Anode Anode Cathode

—/Celgard 2400 bare graphene/— —/PP —/PE film —/— —/Celgard 2300 films Graphene foam/— —/PE film Graphene films/PE film Al/PP Al foil, steel foil/glass fiber —/PP —/Celgard 2400 —/— Al foil/graphene —/Celgard 2400 Metallic foil/glass fiber —/— —/glass fiber —/glass fiber —/glass fiber

28

CD a) (C or mA g-1) DC/CC b) (mA h g-1) 50, 2900/694 C/0.5, 343/258 100, 291/— C/0.5, 158/— —, 5.848 mA h cm-2/— C/0.1, 166.5/— 500, 1551.2/1201.3 C/10, — —, 899/614 50, 1500/— C/0.2, 2278/1150 300, 1278/— 1500, — C/0.1, 1393/— 1500, — C/0.2, 942.4/— 25, —/338 200, 1121/643 100, 676/— 500, 1230/— C/5, 97.6/—

Capacity (mA h g-1)/ Cycles

Flexibility

530, 55 117, 10,000 172, 100 116, 10 2.512 mA h cm-2, 60 165.3, 50 ~1200, 400 ~996, 200 498, 20 760, 200 710, 200 1150, 50 580, 500 689, 50 527, 1000 775.2, 300 230, 15 548, 100 244, 300 523, 100 47.8, 200

Bent, folded Bent Bent Bent Bent Bent Bent Bent Bent Bent, folded Bent Bent Bent Bent Bent Bent Stretchable Bent Bent, rolled Bent Bent

Ref

[174] [175] [176] [177] [178] [179] [180] [181] [182] [183] [184] [185] [186] [187] [188] [189] [190] [191] [192] [193] [194]

a) CD: Current density (C or mA g-1); b) DC/CC: Initial discharge/charge capacity (mA h g-1). EC/DMC: Ethylene carbonate/dimethyl carbonate; DO/DEO: 1,3-dioxolane and 1,2-dimethoxyethane PC/FEC: propylene carbonate and fluoroethylene carbonate; PE: Polyethylene; PP: polypropylene. 1F:1D fibers; 2F: 2D films; 3F: 3D foams; 3N:3D networks. Illustration: Electrolytes: [174–182]: LiPF6/EC/DMC; [183–189]: LiTFSI/LiNO3/DOL/DME; [190,191,193]: NaClO4/EC/DMC; [192]: NaClO4/PC/FEC; [194]: NaClO4/PC/EC. The counter electrodes: [174–189]: Lithium foil; [190–192,194]: Na metal; [193]: NVP/RGO.

3.1 Lithium-ion batteries

However, due to its high operating potential, long cycling life, and relatively simple design, traditional LIBs technology developed in 1990s have since been dominant in practical applications.[195,196] In fact, traditional LIBs have been notorious for two major shortcomings, which have mainly motivated researches into newer battery systems. One of the shortcomings lies in the graphite negative electrode with a theoretical gravimetric capacity of 372 mA h g−1. The other shortcoming includes the limited availability of i in earth’s crust and high cost, which cannot meet the increasing needs in the future years.[197–199] Besides, the market growth of flexible batteries in flexible electronics are increasingly rapid and thus requires good capacity, cycling life, power density, energy density and flexibility of LIBs under deformed states.[200–203] For example, in 2013, Wang et al. adopted self-healing chemistry to Si microparticle (SiMP) anodes to overcome their short cycling life of LIBs.[199] They obtained a cycling life ten times longer than state-of-art anodes made from SiMPs and achieved a high capacity of ∼3,000 mA h g-1 when coated with a self-healing polymer. The self-healing feature is highly desirable for rechargeable batteries due to the life limitation under the mechanical conditions over the cycling process.

29

3.1.1 Pure graphene

According to recent studies, graphene paper (GP) has been a novel promising candidate that can be applied in energy storage systems such as SCs and SIBs. With high mechanical strength and flexibility, it is feasible for the 2D planar graphene to arrange individual sheets into the GP that is fabricated by applying a flow-directed assembly. What is more, using the GP directly as a LIB anode and with neither binder materials nor conducting additives added, the total weight and volume of the electrode can also reduce. In this case, for a device, its whole energy density can be improved in comparison to conventional electrodes. Kim et al. fabricated a highly flexible GP made from graphene nanoplatelets (GNPs) and GO for application in a high-performance, additive-free anode in LIBs.[174] GOs can improve the stability of the composite structure and the capacity of Li storage. Moreover, the obtained GP displayed the increasing graphene interlayer distance and microscopic wrinkles resulting in improved Li storage and good flexibility. The LIB anode based on the GNP/GO paper presented a high specific capacity of 694 mA h g-1, high rate performance and a stable cycling performance. Additionally, a pouch-type flexible battery was assembled and indicated a stable and practical performance even when the battery was bent and folded. The GNP/GO paper remained its original structure and initial conductance over 750 bends, which indicated its good flexibility and potential in flexible LIBs.

3.1.2 Metal oxide/graphene

Now, because of its low theoretical capacity, it is commercially impossible for the graphene anode materials to be applied for high-power use which has been under great demands.[204] 30

Graphene-based materials consisting of graphene and different TMOs such as TiO2, MoO3, SnO2, V2O5 display superior electrochemical performances and morphologies of 2D hybrid films and 3D networks including 3D foams play a critical impact in flexible characteristics of flexible LIBs.[175– 180] For example, Feng et al. synthesized flexible mesoporous TiO2-GS/graphene (mTiO2-GS/G) composite films (Figure 7a).[175] When packed in flexible cells, the mTiO2-GS/G electrode retained good charging/discharging capabilities even when bent and similar rate capabilities when the cell was flat and bent. The capacities of 150 mA h g−1 could be obtained at 10 C under flat and bent conditions (Figure 7b). Moreover, at the 400th cycle, flat and bent composite electrodes still maintain the capacities of 130 mA h g−1 and 150 mA h g−1, respectively (Figure 7c). Besides, two flexible cells in series could power a commercial 3 V LED at 20 C and the brightness of the LED showed no obvious changes when the cells were under bent state. Noerochim et al. prepared flexible and binder-free MoO3 nanobelt/graphene film electrode via a two-step microwave hydrothermal method.[176] The as-obtained hybrid composites demonstrate good rate capability, capacity, and superb cyclability compared to the pure MoO3 film, which delivered a discharge capacity of 291 mA h g-1 for the first cycle at 100 mA g-1 and a capacity of 172 mA h g-1 after 100 cycling tests. Liu et al. adopted a novel lyotropic liquid-crystal (LC) based assembly method to prepare V2O5/rGO composite films.[205] The flexible LIB based on the V2O5/rGO electrodes and Cu sheets as current collectors delivered a capacity of 215 mA h g-1 at 0.1 A g-1 and good flexibility, which can be described to the 2D V2O5/rGO films. Different from above hybrid films, Kim et al. successfully fabricated a freestanding ultrathin 31

graphite (FSG) film with Li4Ti5O12 (LTO) nanowire arrays, which could be used as both the current collector and the anode.[177] The CVD method as the first step showed the growth of FSG films after chemical etching of the Ni foam and the LTO NW arrays were subsequently grown on the FSG films. The FSG films with a thickness of 5 μm were composed of a large amount of stacked graphene layers to make sure that it still maintained freestanding. As a result, the as-fabricated LTO-FSG electrodes exhibited discharge capacities of 154 mA h g-1 and maintained stable even for 500 cycling tests at a 20 C, demonstrating superior cycling performance of the electrodes. In addition, a flexible LIB based on the composite electrodes showed its good flexibility under different bending deformations. Besides, 3D metal oxide/graphene hybrid networks also demonstrate good flexibility. Ding et al. successfully prepared 3D graphene/LiFePO4 structures via solvent evaporation method.[179] The graphene/LiFePO4 nanostructures exhibited good electrochemical properties and superb flexibility. The composites with low graphene content delivered a capacitance of 163.7 mA h g-1 at 0.1 C and 114 mA h g-1 at 5 C. Moreover, the cells based on graphene/LiFePO4 as cathode and lithium metal as anode were tested under different bending deformations with a bending angle of 45o, 90o and 120o, which indicated excellent flexible properties of the cells. Zhang et al. prepared well-dispersed SnO2 NPs anchored on 3D non-woven cotton covered by graphene (CGN/SnO2) as an anode via a facile in situ and thermal annealing process.[178] The SnO2 NPs decorated on the GSs can serve as spacers to reduce the GSs stack. Especially, the anode still maintained a good reversible capacity, cycling stability and rate capability even when bent for many cycles. This method provided a great potential for application in flexible and wearable electronic devices. Sun et al. prepared a coral-like 3D 32

hierarchical heterostructure by using Fe3O4-CoO as a model system.[180] In this structure, Fe3O4 NPs are interspersed on cross-linked CoO NFs which stand vertically on graphene foam. The as-fabricated electrode based on coral-like hierarchical structure as an anode delivered a high reversible specific capacity of ∼1200 mA h g−1 at 0.5 A g−1 for 400 cycling tests. Additionally, the LIBs can even light a green LED when bent and maintained a good capacity when bent, showing the superior flexibility of the battery.

3.1.3 Others

In practical LIBs, graphitic carbon materials are used as anode material, which has been the main constrained factor with unsatisfied theoretical capacity for LiC6 and inferior rate performance.[206] Therefore, it is highly desirable to replace graphitic carbons by high capacity electrode materials, such as alloys (Si and Ge), metal oxides (Co3O4 and GeO2) and metal sulfides (MoS2).[181] Mo et al. successfully fabricated a 3D interconnected porous Ni-doped graphene foam with encapsulated Ge quantum dot/Ni-doped graphene (Ge-QD@NG/NGF) yolk-shell nanostructure.[181] Figure 7d describes the differences between the flexible Ge-QD@NG/NGF/PDMS electrode and the traditional electrode, which shows the importance of the current collector. The composite battery based on Ge-QD@NG/NGF/PDMS electrode as an anode and lithium foil as the counter electrode showed high specific reversible capacity of 1220 mA h g-1, long cycling capability of over 96% reversible capacity retention and good rate performance of over 800 mA h g-1 at 40 C. The bending tests of the Ge-QD@NG/NGF/PDMS yolk-shell-based battery were also investigated, demonstrating a good flexibility of the yolk-shell electrode (Figure 7e,f). Metal oxides/carbon composites with microstructures of 2D films show a great potential in 33

flexible LIBs. Guo et al. designed and fabricated pie-like GP@Fe3O4@carbon film as high-performance integrated electrode for LIBs.[207] Based on the combined structural advantages, the present pie-like electrode with interior array architecture therefore exhibited an outstanding cyclability with a specific capacity of 852 mA g-1 over 1000 cycling tests at 2 A g-1. GP is selected as binder-free integrated substrate, which can accelerate electron transfer, buffer volume changes of Fe3O4, contribute to additional capacity, and endow the electrode with flexibility. Li et al. fabricated a 2D/3D hybrid film structure consisting of 2D rGO sheets and a 3D MnO2-rGO-CNT (MGC) composite through vacuum filtration and thermal annealing approaches.[208] All the components in the 2D/3D thin film anode have a synergistic effect on the improved performance. The initial discharge specific capacity of the electrode with the MnO2 content of 56 wt% achieved discharge capacities of 1656.8 mA h g-1 for the first cycle and 1172.5 mA h g-1 after 100 cycles at 100 mA g-1. Moreover, the flexible LIB indicated good flexible characteristics under deformed conditions.

Figure 7. a) Schematic illustrations for the preparation of TiO2-GS/G composite film and involved mechanisms; b) Rate performance of the mTiO2-GS/G electrode measured by flexible half-cells under flat and bent states; c) Cycling performance at 10 C under flat and bent deformed states. d) Comparison between electrode design in which Ge is coated on the Cu foil and a NGF-based flexible 34

electrode; e,f) Galvanostatic charging/discharging curves and cyclic performance of the battery under flat and bent states. a-c) Reproduced with permission.[175] Copyright 2016, IOP Publishing Ltd. d-f) Reproduced with permission.[181] Copyright 2016, Nature.

It has been proven that in FESDs elementary S proves to be an excellent cathode material for application in LSBs. However, S-based materials can also be applied in flexible LIBs and exhibited excellent electrochemical performances and flexibility. Kumar et al. prepared a graphene/S electrode through a low temperature spraying process.[183] The graphene/S electrode exhibited good coulombic efficiency of ∼98% and high capacity of ∼1500 mA h g−1 along with good cyclability of ∼70% capacity retention after 250 cycling tests. Two types of flexible batteries were assembled based on the composite electrode. One flexible LIB based on graphene-S cathode and Li metal as an anode also can light up a green LED when bent. Another flexible battery based on LiMn2O4 cathode and the graphene-S anode could power a red LED at 1.95 V under a folded deformation.

3.2 Lithium-sulfur batteries

Rechargeable batteries for applications in portable electronic devices and electric vehicles, are ascribed to the high energy densities and long cycle life. However, LIBs cannot well satisfy these requirements. LSBs are one of the promising next-generation energy storage systems with a two-electron reaction, which result in a higher theoretical capacity (1675 mA h g-1) and energy density (2600 Wh kg-1) than that of LIBs.[209–211]

3.2.1 Sulfur/graphene

It has been shown that in FESDs elementary S is an excellent cathode material owing to its low

35

environmental pollution, natural abundance and low cost. LSBs have now focused on the use of low-cost technology and materials so that the material could be applied on a large scale.[209,212– 214] 1D fibrous LSBs endow themselves with superior weavability and flexibility. Liu et al. prepared a flexible fibrous cathode via depositing rGO/sulfur composites onto SSF yarn.[215] The wire-shaped LSB based on the fibrous cathode and, SSF as the current collector indicated superb flexibility and electrochemical properties. The LSB delivered a capacity of 335 mA h g−1 at 167.5 mA g−1 after 100 cycles and powered the LED at different bending angles, which can be described to the fibrous cathodes with their weavability, flexibility and high conductivity. 2D films possess good mechanical flexibility and thus show excellent electrochemical performances in the deformed states. Wu et al. fabricated graphene-based porous carbon (GPC) films with S via a series of processes as shown in Figure 8a.[184] Reversible capacities of the GPC-S films achieve 723, 718, 710 mA h g-1 for the 1st, 100th, 200th cycle. Moreover, the reversible capacity of 647 mA h g-1 and a capacity retention of 89.5% could still be obtained at the 300th cycle as shown in Figure 8c. More importantly, the electrochemical properties of the cathode were also tested when the cathode was under bent deformation (Figure 8b). A discharge capacity of 679 mA h g-1 could be achieved at 0.5 C and 1 C for 20 cycles when the battery was bent, showing a good mechanical flexibility of the device. At the same time, the battery could power the LED when bent and folded, showing their potential application. Different from using a free-standing metal or graphene current collector, a graphene membrane can be directly coated on a commercial separator, which demonstrates good flexibility for application 36

in LSBs.[185] Zhou et al. prepared an integrated structure of S and graphene coated on a polypropylene (PP) separator (S-G@PP) with good flexibility, mechanical behavior and a high energy density for LSBs (Figure 8d).[185] Figure 8e displays a good electrical conductivity of about 800 S m−1, which showed nearly no decay after 50,000 bend tests, demonstrating the integrity of S-G@PP. The charge-discharge curves under both flat and bent states displayed no obvious difference as shown in Figure 8f. The flexible device delivered a capacity of 722 mA h g−1 with a coulombic efficiency of 98% over 30 cycles (Figure 8g).

Figure 8 a.) Synthesis of free-standing GPC films with S; b) The cycling performance for the GPC-S cathode films at 0.5 C and 1 C (inset of a bent cell encapsulated in the glass bottle filled with argon); c) Cycling performance for the GPC films with 41 wt% S at 1 C and their corresponding coulombic efficiency. Reproduced with permission.[184] Copyright 2015, The Royal Society of Chemistry. d) Schematic illustrations for the electrode configuration with an integrated structure of S and G@PP separator and the corresponding battery assembly; e) Stability of an integrated flexible electrode in terms of electrical conductivity for 50,000 cycles; f) GCD curves of the battery under flat and bent 37

states; g) Cycling performance of the battery when bent for 30 cycles at 0.75 A g−1. Reproduced with permission.[185] Copyright 2014, Wiley-VCH.

Besides, 3D graphene porous structure benefits the utilization of S and S anchoring. Lin et al. fabricated a 3D graphene sponge with S NPs via a reduction induced self-assembly process.[186] The composite can be used as a binder-free cathode with good conductivity, structural integrity and flexibility. As a result, compared with the pure graphene sponge electrode, the 3DGS electrode delivered a high reversible capacity of 580 mA h g-1 for 500 cycling tests at 1.5 A g-1 and a high capacity retention of 78.4%. Huang et al. successfully fabricated a self-standing flexible mesoporous graphene paper (MGP) with S immobilization via a sulfur vapor treatment method, which was directly used as LSB electrodes without a binder or conductive additives.[187] It is demonstrated that a discharge capacity of the MGP-S material can reach 1393 mA h g-1 at the 1st cycle much higher than that of bare S (371 mA h g-1). Additionally, the as-obtained composite material exhibited an enhanced cyclability and rate performance for a great potential in flexible LSBs. Zhou et al. prepared a flexible electrode based on graphene foam with S loading via a series of processes as shown in Figure 9a.[188] The composite electrode with different S loadings delivered a superb rate capacity of over 450 mA h g-1 even at 6 A g-1 (Figure 9d). Moreover, the S-PDMS/graphene foam electrode with a large amount of S still maintained its flexibility, which delivered a good electrical conductivity of ~125 S m-1 and showed no obvious change when the electrode was bent for 22000 cycles (Figure 9b,e). As shown in Figure 9c, the structure of the S-PDMS/graphene foam electrode differs from that of traditional one. The composite electrode with different S loadings delivered a superb rate capacity of over 450 mA h g-1 even at 6 A g-1. At the same time, in order to demonstrate 38

the practicability of this FESD, a flexible prototype LSB was assembled, which was capable of litting up a red LED under bent deformation (Figure 9f).

Figure 9. a) Schematic illustrations for preparing PDMS/graphene foam and S-PDMS/graphene foam; b) Photograph of a flexible S-PDMS/graphene foam electrode with 10.1 mg cm-2 S loading; c) Comparison between electrode design in which S is coated on an Al foil and a graphene foam-based flexible electrode; d) Rate cyclability of the S-PDMS/graphene foam electrodes with different S loadings and the electrode with S coated on an Al foil; e) Stability of a S-PDMS/graphene foam electrode with 10.1mg cm-2 S loading in terms of electrical conductivity over 22000 cycles; f) Photograph of a prototype flexible LSB litting up a red LED under bent state. Reproduced with permission.[188] Copyright 2014, Elsevier Ltd.

3.2.2 Li2S/graphene

Recently, studies on Li2S with prospective results have occurred. Owing to its high theoretical capacity (1166 mA h g−1) and melting point (938 °C), anodes based on Li2S have more safety operation. However, S electrodes have some drawbacks such as poor electrical conductivity and intrinsic polysulfide shuttle. And even though Li2S has higher electrical conductivity than pure S, the introduce of carbon materials as additional electron conductors into Li2S is essential. Moreover, in order to utilize Li2S electrodes for LSBs, 2D or 3D framework based on uniform dispersion of Li2S 39

have promoted the electrochemical performances and flexible properties of LSBs.[216] He et al. fabricated 3D Li2S/graphene (3DLG) hierarchical structure via a simple infiltration method.[189] The high rate capacity of the 3DLG up to 4 C achieved 598.6 mA h g-1. The assembled battery based on the 3DLG electrode exhibited good flexibility without structural change when bent. Besides, this novel Li2S battery is capable of lighting a red LED even when seriously bent. He et al. successfully fabricated 3D CNT/graphene-Li2S (3DCG-Li2S) cathodes via facile solvothermal reaction and subsequent liquid infiltration-evaporation coating.[217] The 3D mesoporous structure with 2D GSs and 1D CNTs offers good electrical contact and ionic diffusion, and brings about a low solubility of LiSx in electrolytes in charges/discharges. The flexible 3DCG-Li2S cathode demonstrated excellent electrochemical performances with reversible discharge capacities of 1123.6 mA h g−1 and 914.6 mA h g−1 as well as 0.02% long-term capacity decay per cycle and a high rate capacity of 514 mA h g−1 at 4 C. Even at 300th cycle, a capacity of 958.3 mA h g−1 was still maintained. Hence, the 3DCG-Li2S aerogel has becoming a promising cathode material for flexible LSBs.

3.3 Sodium batteries

Although SIBs are inferior to LIBs in many respects, such as volumetric, power densities and coulombic efficiency, they are being dynamically explored for practical applications owing to the high abundance, low cost of sodium, suitable redox potential and similar intercalation chemistry to Li as shown in Table 2.[167,218,219] Sulfides have also been investigated as potential electrode materials due to their high potential capacity.[220] David et al. successfully fabricated layered freestanding papers composed of MoS 2 40

and rGO flakes, which used as a self-standing flexible electrode in SIBs.[190] The MoS2/rGO papers possessed a capacity of 338 mA h g-1 for the first cycle. Moreover, the electrode exhibited a stable charge capacity of ~ 230 mA h g-1 for superior Na cycling ability. Mechanical tests were also studied, indicating the flexibility of the MoS2/rGO electrodes. Furthermore, the composite paper was directly employed as counter electrode in SIB half-cell, and it showed a potential as an anode for application in a SIB full cell. Xu et al. fabricated a flexible self-standing SnS2@graphene via simple two-step solvothermal method.[191] The flexible nanosheets possessed an enhanced rate performance with discharge capacities of 633, 523, 458 and 348 mA h g-1 at 0.2, 1.0, 2.0, and 3.0 A g-1, respectively. Such a freestanding composite as anodes shows great potential in flexible SIBs. The transition metals and their alloys can easily alloy with Na, which results in higher theoretical Na storage. A variety of transition metals and their alloys, such as Sn, Sb, and their alloy (SnSb), show great potential, because they can alloy with Na, leading to higher theoretical specific capacity of Na storage. Wang et al. fabricated paper-like carbon-coated Na3V2(PO4)3/rGO (NVP@C@rGO) via a facile vacuum filtration method.[194] The symmetric full cell based on NVP@C@rGO electrodes as both an anode and cathode delivered a capacity of 74.1 mA h g-1 at 0.5 C and 47.8 mA h g-1 at 3 C after 200 cycles. Moreover, two pieces of full-cell device in series can easily power a commercial red LED under the straight state, and still work well under deformed conditions. Zhang et al. successfully prepared flexible Sb/rGO and NVP/rGO paper via simple processes (Figure 10a).[193] SEM and TEM images of the Sb/rGO paper are as shown in Figure 10c and d. Both of the obtained composite papers possessed good flexibility and tailorability. Moreover, the flexible Sb/rGO and NVP/rGO electrodes used as an anode and cathode were 41

assembled into a SIB as shown in Figure 10g, which indicated superior cycling stability and high capacity. The initial charge/discharge capacities achieved 642 and 481 mA h g−1 at 100 mA g−1, respectively and a reversible capacity of ~ 400 mA h g

−1

could be obtained over 100 cycles.

Additionally, a commercial red LED based on the flexible SIB was fabricated and lit up even when the battery was bent for 30 cycles (Figure 10e,f). Thus, the flexible Sb/rGO and NVP/rGO paper provides great potential as flexible electrodes for FESDs such as smart electronics, and wearable devices. Different from above, graphene materials can also be applied as flexible substrates for SIBs. Li et al. synthesized PDMS/rGO sponge via a series of processes as shown in Figure 10h, which was employed as the electrode substrates and fabricated an all-stretchable-component sodium-ion full battery based on the PDMS/rGO sponge/VOPO4 composite as a cathode and PDMS/rGO sponge/hard carbon as an anode (Figure 10b).[221] As a result, the as-fabricated stretchable sodium-ion full battery is capable of litting up a commercial blue LED light in the stretched conditions with approximately 50% strains. Moreover, different from other FESDs measured at directly powering the LED lights, a stretchable sodium-ion full battery was fixed on an athletic elbow brace and continuously powered the commercial LED light under different bending deformations (Figure 10i). The battery demonstrated superb flexible performances even under harshly deformed conditions, which can be attributed to the high electrochemical conductivity, stable porous architecture of the PDMS/rGO sponge, as well as the robust mechanical deformability.

42

Figure 10. a) Schematic drawing of the synthesis of the Sb/rGO paper electrode; b) Schematic illustration of the fabricated stretchable sodium-ion full battery; c) SEM image for the cross section of the as-obtained Sb/rGO paper; d) TEM of the Sb/rGO paper; e) Digital picture for the bending sodium-ion full cell that lights a LED; f) Digital picture for the free-bending sodium-ion full cell that lights a LED after 30 bending tests; g) Schematic illustration for the fabrication of the sodium-ion full cell; h) Schematic of preparation steps of conductive PDMS/rGO sponge; i) Photographs of the stretchable sodium-ion full battery mounted on an athletic elbow brace under different bending states. a,c-g) Reproduced with permission.[193] Copyright 2015, Wiley-VCH. b,h,i) Reproduced with permission.[221] Copyright 2017, Wiley-VCH.

4. New-type FESDs

Apart from above SCs, LIBs, LSBs and SIBs, other FESDs such as flexible micro-SCs, micro-scale batteries, Li-O2 batteries and Al-ion batteries also show a great potential in FESDs. Recently, many efforts have been made for fabricating flexible micro-SCs for FESDs.[222] Fan et al. fabricated 1D fibers via thermal reducing process, which showed tensile strength of ~729 MPa even after 2800 °C

43

thermal annealing treatment.[81] Thus the flexible fiber-based micro-SC using PET substrate also exhibited superb flexibility of 93% capacitance retention when bent 90° for 2000 cycles. Besides, Yu et al. fabricated single-walled carbon nanotube (SWNT)/nitrogen-doped rGO fibers by using a fused-silica capillary column as a hydrothermal microreactor (Figure 11a,b).[83] The as-obtained dry hybrid fibers possessed superb tensile strength of 84-165 MPa, and can be bent into different shapes or woven into textile structures. Furthermore, a micro-SC based on CNTs-GFs, PET substrate and PVA/H3PO4 electrolyte was assembled (Figure 11c) and showed good cyclability of 93% capacitance retention for 10,000 cycling tests. A high volumetric energy density of ~6.3 mWh cm-3 and a maximum power density of 1085 mW cm-3 were obtained. At the same time, it maintained a capacitance retention of >97% after bending 1,000 times at 90o (Figure 11e), demonstrating a high potential in FESDs. Wu et al. successfully fabricated micro-SCs based on the 2D heterostructure film of ultrathin Thiophene and electrochemically exfoliated graphene nanosheets (TP/EG) (Figure 11g,h).[85] The micro-SCs exhibited good rate capability working well up to 1000 V s −1 and superb flexibility under deformed states (Figure 11i). El-Kady et al. fabricated an LSG micro-SC by direct writing of graphene patterns on GO films as shown in Figure 11d.[87] More than 100 micro-SCs can be produced on a single disc in 30 min or less. As shown in Figure 11f, the devices based on flexible substrates demonstrate superb flexible properties. Besides, there are few reports on flexible sodium-ion pseudo-capacitors and other batteries. Dong et al. fabricated a novel FESD based on Na2Ti3O7 nanosheet arrays/carbon textiles (NTO/CT) as anode and rGO film as cathode.[223] The sodium-ion pseudo-capacitors delivered a maximum practical volumetric energy density of 1.3 mWh cm−3 and power density of 70 mW cm−3. 44

Additionally, the flexible device showed a good cycling stability with nearly 100% capacitance retention under deformed states. Zhang et al. utilized laminated cross-linked nanocellulose/GO composites as an electrolyte to fabricate flexible Zn-air batteries.[224] Liu et al. prepared flexible Ni/Fe batteries based on graphene foam/CNTs hybrid films as current collectors, exhibiting high energy/power densities of 100.7 Wh kg-1 at 287 W kg-1 and 70.9 Wh kg-1 at 1.4 kW kg-1.[51] Generally, the mature development of LIBs, LSBs, SIBs with good electrochemical and flexible performances facilitate the processes towards flexible energy storage systems compared to other batteries such as Al-ion batteries. Besides, the flexible requirements of FESDs limited the development of others batteries and thus the articles focusing on other flexible graphene-based batteries such as Al-ion batteries and Li-O2 batteries are almost none. However, we believe other FESDs including flexible micro-SCs, micro-scale batteries, novel Al-ion batteries and Li-O2 batteries also have a great potential in FESDs in the near future due to the flexible structures (1D graphene fibers, 2D graphene films, 3D graphene networks), flexible current collectors or substrates, and tailored electrolytes.

45

Figure 11. a,b) Schematic illustration of the preparation of carbon hybrid microfibers; c) Schematic illustration of a micro-SC; d) Schematic illustration of the fabrication for an LSG micro-SC; e) Capacitance retention after 1,000 cycles with a bending angle of 90o; f) A photograph of LSG-micro-SC bent with a tweezers showing good flexibility; g) SEM image of the freestanding TP/EG heterostructure film; h) The top-view SEM image of the TP/EG heterostructure films with high magnification, showing the presence of the wrinkles from graphene; i) CV curves of TP/EG micro-SCs obtained at 1 V s-1 under flat and bending states. a-c,e) Reproduced with permission.[83] Copyright 2014, Macmillan Publishers Limited. d,f) Reproduced with permission.[87] Copyright 2013, Macmillan Publishers Limited. g-i) Reproduced with permission.[85] Copyright 2016, WILEY-VCH.

5. Conclusions and Outlook

This review provides an updated overview of graphene-based materials for application in FESDs including flexible SCs, LIBs, LSBs and SIBs. Graphene with its unique atom-thick 2D structure has been extensively studied as electrode materials for FESDs owing to their high SSA, superb mechanical, electrical and electrochemical behaviors. Therefore, various graphene-based electrodes 46

of FESDs have been developed. In order to fulfill the industrial demands of FESDs, lightweight FESDs can be applied in compressible SCs under harsh conditions, for example, flying devices, electric vehicles, transparent flexible devices including wearable displays and touch-screens. Obviously, there is still much room for FESDs developed in the laboratories to be mass produced on an industrial level. Owing to the ever-increasing requirement of FESDs, design and engineering of revolutionary materials are to be promoted. In many cases, however, the material and device properties need to be optimized for further improvement. The certain challenges still need to be overcome and considered for fabricating high-performance and cost-effective FESDs. Firstly, the selection of electrode materials is critical. The limitations of restacking GSs may result in a great decrease in the performances of FESDs. In order to achieve desired performances of graphene-based FESDs, the structures of 1D fibers, 2D films and 3D networks (foams) with large SSA, and robust mechanical strength show great advantages in the electrochemical performances and flexible performances. Secondly, the flexible substrates and electrolytes also of great importance. Flexible substrates may serve as both current collectors and flexible electrode supports, which endow the devices with flexibility. Electrolytes such as PVA/H3PO4, H2SO4-PVA can effectively protect the FESDs from current leaks and tolerate the mechanical stresses. The future directions of graphene-based FESDs are as follows. 1) Novel nanostructured materials for flexible electrodes are required along with suitable substrates and electrolytes for FESDs; 2) Graphene-based FESDs need to suffer from certain mechanical loading conditions without greatly sacrificing electrochemical performances; 3) FESDs are to be more widely applied in our daily applications including portable devices, vehicles, medical facilities and flying devices. For 47

now, rapid progress has been made for graphene-based FESDs with good electrochemical and flexible properties. It is anticipated that graphene-based nanocomposites will have a wide application in high-performance FESDs. Acknowledgements This work was supported by the Program for New Century Excellent Talents of the University in China (NCET-13-0645) and the National Natural Science Foundation of China (NSFC-21201010, 21671170 and 21673203), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (164200510018), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN004), the Six Talent Plan (2015-XCL-030), and Qinglan Project. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University.

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Biography

Xiaotian Guo is now a graduate student under Prof Xue's supervision, Yangzhou University of chemistry and chemical engineering, China. Her research mainly focuses on the field of electrochemical energy storage materials and their applications for supercapacitors.

Huaiguo Xue received his Ph.D. degree in polymer chemistry from the Zhejiang University in 2002. He is currently a professor of physical chemistry and the dean of the College of Chemistry and Chemical Engineering at the Yangzhou University. His research interests focuses on electrochemistry, functional polymer and biosensors.

Huan Pang received his Ph. D. degree from Nanjing University in 2011. He then founded his research group in Anyang Normal University where he was appointed as a distinguished professor in 2013. He has now jointed Yangzhou University as a university distinguished professor. He has published more than 120 papers in peer-reviewing journals including Chemical Society Reviews, Advanced Materials, Energy Environ. Sci., with 3900 citations (H-index=33). His research interests include the development of inorganic nanostructures and their applications in flexible electronics with a focus on energy devices.

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