Mn3O4 Nanocomposite Membrane as

Highly Flexible Graphene/Mn3O4 Nanocomposite Membrane as Advanced Anodes for Li-Ion Batteries Jian-Gan Wang,† Dandan Jin,† Rui Zhou,† Xu Li,‡ Xing-rui Liu,† Chao Shen,† Keyu Xie,† Baohua Li,‡ Feiyu Kang,‡ and Bingqing Wei*,†,§ †

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China ‡ Engineering Laboratory for Functionalized Carbon Materials and Shenzhen Key Laboratory for Graphene-based Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China § Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Advanced electrode design is crucial in the rapid development of flexible energy storage devices for emerging flexible electronics. Herein, we report a rational synthesis of graphene/Mn3O4 nanocomposite membranes with excellent mechanical flexibility and Li-ion storage properties. The strong interaction between the large-area graphene nanosheets and long Mn3O4 nanowires not only enables the membrane to endure various mechanical deformations but also produces a strong synergistic effect of enhanced reaction kinetics by providing enlarged electrode/electrolyte contact area and reduced electron/ion transport resistance. The mechanically robust membrane is explored as a freestanding anode for Li-ion batteries, which delivers a high specific capacity of ∼800 mAh g−1 based on the total electrode mass, along with superior high-rate capability and excellent cycling stability. A flexible full Li-ion battery is fabricated with excellent electrochemical properties and high flexibility, demonstrating its great potential for high-performance flexible energy storage devices. KEYWORDS: graphene, Mn3O4, nanocomposite, flexible Li-ion batteries, high performance critical issue of fabricating a flexible LIB device is the advanced design of flexible electrode materials with good mechanical deformations (e.g., bending, twisting, and even folding) and high electrochemical performance (e.g., high specific capacity, high rate capability, and long cycle life). The design strategies can be categorized as (i) freestanding and flexible electrode materials without using binders and conductive additives, such as carbon nanotubes (CNT), graphene, and their composites,8−11 and (ii) electrode materials grown or coated on

F

lexible smart electronics have become an emerging and promising technology in a myriad of applications such as the pioneering products of Samsung flexible prototype display windows and Philips Fluid flexible smartphone.1,2 The rapid development in this field has simultaneously provoked ever-increasing research interests in the corresponding electrochemical energy storage systems. Flexible electronics require the energy storage devices to be mechanically flexible/ bendable/wearable, portable, lightweight, and of high-performance.3 To date, considerable effort has been made to fabricate flexible and even stretchable energy storage devices.2,4−7 Li-ion batteries (LIBs) are of particular interests due to their high energy density, high operating voltage, and long lifetime. The © 2016 American Chemical Society

Received: April 6, 2016 Accepted: May 12, 2016 Published: May 12, 2016 6227

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Figure 1. Schematic diagram (top view and cross-sectional view) of the rGO/Mn3O4 membrane and its digital photos illustrating the flexibility (bending, rolling, twisting) and foldability (1-, 2-, 3-fold) states.

boost electron flow throughout the electrode. Third, the nanosized structures can shorten electron/Li+ transport distances for enhanced reaction kinetics. Finally, the porous architecture facilitates easy access of electrolyte ions to the electrode surfaces. As a result, the flexible rGO/Mn3O4 nanocomposite can deliver a high specific capacity of ∼800 mAh g−1 at 100 mA g−1 based on the total weight of the electrode, together with excellent rate capability and cycling stability.

flexible substrates, such as carbon cloth, cellulose paper, and textiles.12−14 It should be noted that an ideal flexible electrode material should be freestanding without the assistance of inactive mechanical support, and be sufficiently conductive for efficient electrochemical utilization of each active component.15−17 Therefore, although flexible electrode materials have been substantially reported, it is still urgent and challenging to develop a flexible, inexpensive, lightweight, and high-performance electrode material. Transition metal oxides are attractive anode substitutes of conventional graphite. In particular, Mn3O4 has been part of extensive research activities due to its high theoretical specific capacity of 936 mAh g−1, small electromotive force, low cost, natural abundance of Mn on earth, and small environmental footprints in both synthesis and applications.18−20 However, the use of Mn3O4 is significantly inhibited by (i) poor electrical conductivity (10−7 ∼ 10−8 S cm−1) that kinetically limits rate capability and (ii) large volume change that renders rapid capacity loss during charge/discharge processes.18,21 To tackle these problems, the most useful approach is to construct Mn3O4 nanostructures with a carbonaceous matrix.18,19,21−25 Graphene is an excellent matrix candidate because of its high conductivity, large surface area, good chemical stability, and strong mechanical strength.26,27 To this end, graphene/Mn3O4 nanoparticles have shown outstanding Li-storage performance in previous studies.18,28 However, the powder-like graphene/ Mn3O4 nanocomposites cannot meet the robust, flexible requirements of LIBs. In this work, we report a freestanding and lightweight graphene/Mn3O4 nanocomposite membrane with high mechanical flexibility and foldability. Figure 1 schematically illustrates the typical membrane structure, in which long onedimensional (1D) Mn3O4 nanowires and two-dimensional (2D) reduced graphene oxide (rGO) nanosheets are integrated to construct a robust 1D/2D hybrid architecture using a simple vacuum filtration method. The as-obtained rGO/Mn3O4 membrane can sustain various mechanical deformations, such as bending, rolling, twisting, and especially, folding multiple times (Figure 1). In this architecture, the 1D nanowires are tangled with graphene nanosheets, which may lead to a strong synergistic effect. First, the nanowires can function as a spacer to prevent adjacent graphene from restacking and agglomeration, and thus enlarge the electrode/electrolyte interface. Second, graphene can serve as a strain cushion to alleviate large volume change of Mn3O4, and also as a conductive network to

RESULTS AND DISCUSSION As described in Methods, all of the samples were prepared by a simple hydrothermal process. The morphology and microstructure of the products are examined by SEM and TEM imaging. Figure 2a shows a panoramic top-view of the rGO/

Figure 2. (a and b) SEM, (c) TEM, and (d) HRTEM (inset: SAED pattern) images of the rGO/Mn3O4 nanocomposite.

Mn3O4 nanocomposites, which are composed of long uniform Mn3O4 nanowires and the transparent, large area, and continuous graphene nanosheets. The length of Mn3O4 nanowires ranges from several tens to hundreds of micrometers, and the size of the graphene nanosheets is about tens of micrometers in width. In addition, both components are homogeneously entangled with each other, thereby construct6228

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Figure 3. (a) XRD patterns and (b) Raman spectrum of the Mn3O4/rGO, Mn3O4, and rGO.

Figure 4. (a) CV, (b) charge/discharge curves, (c) cycling performance and (d) rate capability of the rGO/Mn3O4 nanocomposite. (e) SEM image of the rGO/Mn3O4 nanocomposite after 100 cycles. (f) Energy storage characteristics of the 1D/2D hybrid nanoarchitecture.

ing a porous and interconnected network structure with substantial mechanical strength. As shown in Figure 1, the

nanocomposite stands as a freestanding membrane without the assistance of any binder. The mechanical stability is preserved 6229

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oxidation combustion of carbon in the atmosphere. Hence, the rGO content is estimated to be approximately 32 wt %. The surface elements and their chemical bonding states of the rGO/Mn3O4 nanocomposite were examined using XPS. The survey spectrum in Figure S6 shows the existence of Mn, O, and C elements with no trace of other impurities. The C 1s peak can be resolved into three components centered at 284.3, 285.6, and 288.1 eV, which represent sp2 carbon structure, defects on the rGO due to the presence of carboxylic and hydroxyl functions, and CO bonds, respectively.25,36,37 The result indicates that the functional groups in GO are substantially removed after the hydrothermal treatment. As for the Mn 2p peak (Figure S6c), the Mn 2p1/2 subpeak appears at 653.1 eV, and the Mn 2p3/2 subpeak can be well-resolved into the Mn4+ 2p3/2 part at 642.7 eV and Mn2+ 2p3/2 part at 641.0 eV. The Mn 2p doublet with a spin-energy separation of 11.9 eV is characteristic of Mn3O4.36 Figure S6d displays the deconvoluted O 1s spectrum, where three components located at 529.9, 530.9, and 532.5 eV correspond to Mn−O−Mn bond for oxide, Mn−O−H for hydroxide, and H−O−H for residual water, respectively.21 These results indicate that the chemical states of the graphene and Mn3O4 are in good accordance with the results of XRD and Raman. The rGO/Mn3O4 nanocomposite was directly used as freestanding anodes to evaluate its electrochemical performance. Figure 4a exhibits the cyclic voltammetry (CV) curves of the initial five cycles at a scan rate of 0.2 mV s−1. In the first cathodic cycle, the broad peak at 0.8 V results from the formation of solid-electrolyte-interface (SEI) film, some side reactions between Li+ and active materials, and a concomitant initial reduction of oxide (e.g., Mn3O4 → MnO).18,33 Meanwhile, the sharp peak at a lower potential of 0.08 V corresponds to the following reduction of oxide to metallic Mn (eq 1). From the second and onward cycles, the reduction peak shifts to a higher potential of 0.2 V due to the improved reaction kinetics after the first lithiation.38,39 The anodic peak centered around 1.35 V in the reverse sweep is ascribed to the reoxidation of metallic Mn to the oxide counterpart. It is observed that the cycles are almost overlapped with each other, indicating good electrochemical reversibility and cycling performance. Thus, the reversible conversion reaction between Li+ and Mn3O4 can be expressed as follows:

after various deformations of bending, rolling on a 4 mm rod, twisting, and even folding three times, implying the membrane is applicable as a platform for flexible and even foldable energy storage devices. Moreover, the cross-sectional image (Figure S1) exhibits the homogeneous interdispersion of Mn3O4 nanowires and graphene nanosheets throughout the entire membrane, which is further confirmed by the uniform distribution of the Mn, O and C elements shown in the EDX elemental mapping (Figure S2). The consistent feature of the nanocomposite is ascribed to the presence of GO, which can function as a surfactant to assist the dispersion of Mn3O4 nanowires.10,31,32 A close SEM observation in Figure 2b indicates that the graphene nanosheets are tightly attached to the nanowire surface, and in turn, the nanowires sandwiched in between the nanosheets can prevent their restacking, curling, and aggregating. This unique nanoarchitecture can afford abundant pores and channels for ion transport, allowing more efficient contact between electrolyte and graphene throughout the membrane. Moreover, it is observed that there are many intrinsic wrinkles and ripples on the graphene surfaces (Figure 2b), which can offer more active sites for Li-insertion and more flexibility for membrane deformation. Hence, the outstanding flexibility and foldability of the nanocomposite paper can be ascribed to a collective benefit from the 1D/2D nanowire/ nanosheet architecture. Figure 2c shows the TEM image of the nanocomposites. As observed, the Mn3O4 nanowires are supported onto the graphene nanosheets. The short nanowire segments may probably result from the ultrasonic treatment during the preparation of the TEM samples. The straight nanowires are 100−200 nm in diameter. The HRTEM image of the Mn3O4 nanowires (Figure 2d) shows distinct lattice fringes of 0.49 nm in d-spacing, which is in good agreement with the interplanar distances of (101) planes of Mn3O4 phase. The corresponding selected area electron diffraction (SAED) pattern (inset) displays a set of well-defined diffraction spots, indicating the Mn3O4 nanowire is of high-quality single crystalline nature. For comparison, the pristine Mn3 O4 nanowires are shown in Figure S3. The phase structure of the as-prepared samples was characterized using XRD. As shown in Figure 3a, the main diffraction peaks can be well indexed to hausmannite Mn3O4 phase with a tetragonal spinel structure (PDF #: 24-0734).21,33 The broad peak at 2θ around 24° with a relatively low intensity originates from the (002) plane of rGO. Figure S4 clearly indicates the evolution of the (002) diffraction peak during the hydrothermal GO-to-rGO reduction. To further confirm the structural information, Raman spectra were recorded in the samples (Figure 3b). It is clearly observed that the dominant scattering peak at 652 cm−1 and the minor peaks at 316 and 369 cm−1 are characteristic of the Mn−O vibration modes in the crystalline Mn3O4.19,33,34 In addition, the broad Raman bands located at 1350 and 1590 cm−1 are associated with the A1g vibration mode of the disordered carbon (D bond) and the E2g vibration mode of the defined graphitic carbon (G bond), respectively.21,34,35 The small ID/IG intensity ratio (1.0) indicates a high graphitization degree of rGO, which is beneficial in improving the electrical conductivity of the nanocomposite. The rGO content in the nanocomposite was determined using thermogravimetric analysis (TGA, Figure S5). The initial weight loss before 250 °C can be attributed to the removal of physical and chemical absorbed water. The significant weight loss at around 300 °C is related to the

Mn3O4 + 8Li+ + 8e ↔ 3Mn + 4Li 2O

(1)

The conversion reaction can also be confirmed by the galvanostatic charge/discharge curves. As shown in Figure 4b, the discharge plateau at 0.2 V in the first cycle is associated with the reduction of oxide to metallic Mn, the potential of which increases to 0.44 V in the following cycles. The well-defined charge plateau at around 1.4 V is resulting from the regeneration of oxide from metallic Mn. Clearly, the electrochemical behavior and the voltage plateaus match well with the peak position observed in the CV curves. Figure 4c exhibits the cycling performance of the rGO/ Mn3O4 nanocomposite and the controlled samples of pure Mn3O4 and rGO nanosheets at 100 mA g−1. The first discharge and charge capacities of the nanocomposite are 1271 and 802 mAh g−1, respectively, corresponding to an initial Coulombic efficiency (CE) of 63.1%. The large irreversible capacity loss is due to the inevitable SEI film formation on the electrode surface and some possible side reactions between Li+ and the residual functional groups in the rGO structure. The CE value, however, significantly increases to over 98.2% from the second 6230

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Figure 5. (a) 3D schematic configuration of a flexible rGO/Mn3O4∥LiMn2O4 full battery and optical images showing a red LED successfully powered by the battery under flat and bending states. (b) Charge/discharge curves and (c) cycling performance of a full battery.

Mn3O4 electrode almost lose the Li-ion storage capability at this high C-rate. When the current density is recovered to 100 mA g−1, a high specific capacity of 830 mAh g−1 is regained, again indicating the excellent electrochemical reversibility and cycling stability. To confirm the structural stability upon cycling, the electrode was disassembled after 100 cycles for SEM observation. As shown in Figure 4e, the 1D Mn3O4 nanowire network is preserved without visible structural damage. Figure 4f illustrates the energy storage characteristics of the unique 1D/2D hybrid nanoarchitecture to rationalize the excellent Li-ion storage performance, including high specific capacity, good cycling, and high-rate capability. First, the intimate interaction between Mn3O4 nanowires and graphene nanosheets enables Mn3O4 more electrochemically active, since the highly conductive graphene can efficiently and rapidly collect and migrate charge carries back and forth from the Mn3O4.18 Second, the 1D nanowires sandwiched in between adjacent graphene nanosheets suppress their restacking and aggregating, thus enlarging the electrode/electrolyte interface area for energy storage. Third, the wrinkled graphene nanosheet overcoat can act as an elastic 3D cushion framework to accommodate large volume variation of the Mn3O4 nanowires during lithiation/delithiation processes and maintains the structural integrity of the electrode. Fourth, the nanosized structure can minimize the electron/Li+ transport pathways to enhance the reaction kinetics. Fifth, the freestanding electrode eliminates the use of insulating binders and inactive conductive additives, which avoids dead volume and invalid sites in the electrode. Finally, the 3D porous architecture facilitates fast electrolyte infiltration and ingress throughout the entire electrode. All of these structural characteristics endow the nanocomposite with strong synergistic effect to harvest outstanding Li-ion storage performance. Furthermore, the rGO/Mn3O4 nanocomposite membrane can

cycle. In sharp contrast, both the reversible capacity and the initial CE of nanocomposite electrode are much higher than those of pure Mn3O4 (171 mAh g−1 and 37.4%) and rGO (441 mAh g−1 and 42%). More importantly, the nanocomposite still delivers a high reversible capacity of 702 mAh g−1 after 100 cycles, indicating a good capacity retention of 87.5% that greatly surpasses the controlled electrodes (i.e., 25.8% for Mn3O4 and 69.1% for rGO). The higher specific capacity and better cycling stability of the nanocomposite suggest a strong synergistic effect between the conductive rGO and highcapacity Mn3O4 component. It is worth pointing out that all of the capacity results are calculated based on the total mass of the electrodes. Even so, our capacity value (802 mAh g−1 at 100 mA g−1) is comparable to the rGO/Mn3O4 nanoparticle (810 mAh g−1 at 40 mA g−1)18 and sponge-like Mn3O4 nanoparticle (869 mAh g−1 at 30 mA g−1),33 and also is advantageous to Mn3O4/C nanorods (723 mAh g−1 at 40 mA g−1),21 Mn3O4/ ordered mesoporous carbon (773 mAh g−1 at 100 mA g−1),24 superaligned CNT/Mn3O4 nanoparticles (701.4 mAh g−1 at 93.6 mA g−1)23 and CNT/Mn3O4 nanocrystals (709 mAh g−1 at 100 mA g−1).40 It is well-known that, for the conventional electrodes made by powder-like materials, the electrochemically inert components, such as conductive additives, insulating binders, and current collectors, would account for at least 30− 50 wt % of the total electrode mass. Therefore, our results would be much more intriguing when taking all of the components into account. To further validate the advantage of combining rGO and Mn3O4, the electrodes were measured at different current densities from 100 to 2000 mA g−1. As shown in Figure 4d and Figure S7, the binder-free nanocomposite electrode shows superior rate capability compared to the controlled samples. It is important to note that the hybrid electrode can retain a specific capacity of 308 mAh g−1 even at a high current rate of 2000 mA g−1 (2.14 C, 1 C = 936 mAh g−1), whereas the pure 6231

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transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 160 °C for 8 h. A brown precipitate was obtained after the solution was naturally cooled down to room temperature. The products were filtered and rinsed with deionized water for several times, and finally dried at 80 °C for 12 h. Subsequently, a certain amount of MnO2 nanowires (e.g., 50 mg) were well dispersed in 40 mL of graphene oxide solution (GO, concentration: 1 mg mL−1), which was prepared using a modified Hummer’s method.10,18 The homogeneous suspension was then kept at 180 °C in an oven for 12 h, during which time the GO and MnO2 were reduced to rGO and Mn3O4 under the hydrothermal condition, respectively. The final product was washed several times, and then filtered onto a 0.8-μm porous filter paper via a simple vacuum filtration method. The circular membrane (∼5 cm) was peeled from the filter paper to obtain the flexible and foldable nanocomposite with an areal mass density of 1−1.2 mg cm−2. The thickness of the membrane was about 12 μm (Figure S1). For the control samples of Mn3O4 and rGO, the pristine MnO2 and GO were individually treated under the same hydrothermal process. Material Characterizations. X-ray powder diffraction (XRD, Cu Kα radiation (λ = 1.5418 Å), X’Pert PRO MPD, Philips) was used to analyze the crystal structures. Raman spectrum was collected using a Renishaw Invia Raman microscope (laser wavelength: 514.5 nm). The rGO content in the nanocomposite was determined based on the thermogravimetric analysis (TGA, Mettler Toledo TGA/DSC 3+). The microstructure and morphology were observed by field-emission scanning electron microscopy (FE-SEM, FEI Nano SEM 450) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30G2). The elemental mapping was obtained using energy dispersive X-ray spectroscopy. The surface composition and chemical states of the nanocomposite were investigated using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific). Electrochemical Measurements. The electrochemical properties were measured using coin-type (CR2032) half cells. Both rGO/ Mn3O4 and Mn3O4 working electrodes were punched directly from the as-fabricated membranes, which were used as freestanding electrodes for cell assembly. The working electrodes were assembled in an argonfilled glovebox with metal Li foils as the counter electrode, Celgard 2320 as the separators, and a solution of 1 M LiPF6 in ethylene carbonate (EC)/diemethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1 by volume) as the electrolyte. To assemble a flexible full battery, a 15 × 15 mm2 rGO/Mn3O4 anode was coupled with a LiMn2O4 cathode, which was prepared by coating conventional LiMn2O4 nanoparticle slurry on an Al foil (mass loading: 3.5 mg cm−2) as reported in our previous work.29 Prior to the assembly, the rGO/ Mn3O4 should be prelithiated by placing it in direct contact with a Li foil wet by the electrolyte for 6 h to compensate its large initial irreversible capacity.30 Solartron electrochemical workstation (1260 + 1287, England) was employed to measure cyclic voltammetry (CV) curves in the range of 0.005−3 V at a rate of 0.2 mV s−1. The galvanostatic charge/discharge tests were carried out using a Land Battery Testing system (Land, China).

endure severe shape deformations that may potentially meet the mechanical requirements of flexible and even foldable LIBs. By using the rGO/Mn3O4 membrane as a flexible anode, we further construct a flexible full battery to demonstrate its practical feasibility. Figure 5a exhibits the flexible battery configuration, which is composed of rGO/Mn3O4 as the anode and LiMn2O4/Al foil as a cathode. The optical images illustrate that the as-assembled battery can successfully power a red lightemitting-diode (LED) under both flat and bending states, indicating its flexibility for practical use. Figure 5b presents the charge/discharge curves of the battery cycled at 100 mA g−1 in the voltage window of 2.0−4.1 V. It is known that the total capacity of rGO/Mn3O4 anode (about 1.8 mAh) is much higher than that of cathode (about 0.8 mAh). Hence, the specific capacity is calculated based on the LiMn2O4 mass because the battery is cathode-limited.13,30,41 Encouragingly, the battery can deliver an initial reversible specific capacity of about 100 mAh g−1. In addition, the battery exhibits good cycling performance under flat and bending states with an acceptable specific capacity of 79 mAh g−1 being retained after 100 cycles (Figure 5c). It is believed that the strong membrane integrity helps to maintain the electrochemical properties of the hybrid electrode. To better demonstrate it, the morphology of the folded membrane was characterized. As shown in Figure S8, the microstructure is well preserved without cracks on the membrane folds, indicating the robust mechanical stability of the membrane. Finally, it is worthy to point out that, to meet repetitive bending cycles, the development of a flexible cathode material is equally important to build a practical and flexible battery with high specific capacity and excellent cycling stability.

CONCLUSIONS An advanced flexible and foldable nanocomposite membrane has been successfully fabricated by combining 2D graphene nanosheets and 1D Mn 3 O4 nanowires to construct a mechanically robust entity. The unique 1D/2D hybrid nanoarchitecture can generate a strong synergistic interaction by fully utilizing the highly conductive and elastic graphene and the high-capacity Mn3O4. When used as a freestanding anode for Li-ion batteries, the nanocomposite manifests excellent electrochemical properties including a high specific capacity of 802 mAh g−1 at 100 mA g−1 based on the total electrode mass, a high-rate capacity of 308 mAh g−1 at a current density of 2000 mA g−1, and good cycling stability (13.5% decay in capacity after 100 cycles). Furthermore, the flexible graphene/ Mn3O4∥LiMn2O4 full battery shows a specific capacity of ∼100 mAh g−1 with good cycling stability and high flexibility. The unique electrode structure design may find a promising application in flexible energy storage devices with both high electrochemical performance and excellent mechanical flexibility.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02319. Cross-sectional images, EDX mapping, TGA, XPS, and charge/discharge curves of rGO/Mn3O4. SEM images of Mn3O4 and folded membrane. XRD of GO and rGO (PDF)

METHODS The chemical reagents of manganese(II) sulfate (MnSO4, ≥99%), potassium chlorate (KClO3, ≥99.5%), potassium acetate (CH3COOK, ≥ 92%), acetic acid (CH3COOH, ≥99.5%), and natural graphite powder (≥99.85%) were purchased from Sinopharm Chemical Agent, Co. Ltd., which were of analytical grade and were used as received without purification. Deionized water (18.2 MΩ) was obtained from Mili-Q water purification system (TGI Pure Water Systems, USA). Materials Synthesis. First, ultralong MnO2 nanowires were prepared based on our previous work.38 Typically, a transparent aqueous solution (40 mL) containing MnSO4 (2 mmol), KClO3 (3.5 mmol), CH3COOK (3.5 mmol), and CH3COOH (1.6 mL) was

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6232

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ACKNOWLEDGMENTS The authors acknowledge the financial supports of this work by the National Natural Science Foundation of China (51402236, 51472204, 53102219, 51221001, 51521061), the Natural Science Foundation of Shannxi Province (2015JM5180), the Fundamental Research Funds for the Central Universities (3102014JCQ01020 and 3102015BJ(II)MYZ02), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.: 123-QZ-2015) and the Program of Introducing Talents of Discipline to Universities (B08040). REFERENCES (1) Shao, Y.; El-Kady, M. F.; Wang, L. J.; Zhang, Q.; Li, Y.; Wang, H.; Mousavi, M. F.; Kaner, R. B. Graphene-based Materials for Flexible Supercapacitors. Chem. Soc. Rev. 2015, 44, 3639−3665. (2) Zhou, G.; Li, F.; Cheng, H.-M. Progress in Flexible Lithium Batteries and Future Prospects. Energy Environ. Sci. 2014, 7, 1307− 1338. (3) Sun, Y.; Sills, R. B.; Hu, X.; Seh, Z. W.; Xiao, X.; Xu, H.; Luo, W.; Jin, H.; Xin, Y.; Li, T.; et al. Y. A Bamboo-Inspired Nanostructure Design for Flexible, Foldable, and Twistable Energy Storage Devices. Nano Lett. 2015, 15, 3899−3906. (4) Lu, X.; Yu, M.; Wang, G.; Tong, Y.; Li, Y. Flexible solid-state supercapacitors: design, fabrication and applications. Energy Environ. Sci. 2014, 7, 2160−2181. (5) Liu, Q.-C.; Li, L.; Xu, J.-J.; Chang, Z.-W.; Xu, D.; Yin, Y.-B.; Yang, X.-Y.; Liu, T.; Jiang, Y.-S.; Yan, J.-M.; et al. Flexible and Foldable Li− O2 Battery Based on Paper-Ink Cathode. Adv. Mater. 2015, 27, 8095− 8101. (6) Xie, K.; Wei, B. Materials and Structures for Stretchable Energy Storage and Conversion Devices. Adv. Mater. 2014, 26, 3592−3617. (7) Li, X.; Gu, T.; Wei, B. Q. Dynamic and Galvanic Stability of Stretchable Supercapacitors. Nano Lett. 2012, 12, 6366−6371. (8) Wang, B.; Li, X.; Zhang, X.; Luo, B.; Jin, M.; Liang, M.; Dayeh, S. A.; Picraux, S. T.; Zhi, L. Adaptable Silicon Carbon Nanocables Sandwiched between Reduced Graphene Oxide Sheets as Lithium Ion Battery Anodes. ACS Nano 2013, 7, 1437−1445. (9) Luo, S.; Wang, K.; Wang, J.; Jiang, K.; Li, Q.; Fan, S. Binder−Free LiCoO2/Carbon Nanotube Cathodes for High-Performance Lithium Ion Batteries. Adv. Mater. 2012, 24, 2294−2298. (10) Lee, J. W.; Lim, S. Y.; Jeong, H. M.; Hwang, T. H.; Kang, J. K.; Choi, J. W. Extremely Stable Cycling of Ultra−thin V2O5 Nanowire− Graphene Electrodes for Lithium Rechargeable Battery Cathodes. Energy Environ. Sci. 2012, 5, 9889−9894. (11) Chen, R.; Zhao, T.; Wu, W.; Wu, F.; Li, L.; Qian, J.; Xu, R.; Wu, H.; Albishri, H. M.; Al-Bogami, A. S.; El-Hady, D. A.; Lu, J.; Amine, K. Free-Standing Hierarchically Sandwich−Type Tungsten Disulfide Nanotubes/Graphene Anode for Lithium-Ion Batteries. Nano Lett. 2014, 14, 5899−5904. (12) Shen, L.; Ding, B.; Nie, P.; Cao, G.; Zhang, X. Advanced Energy−Storage Architectures Composed of Spinel Lithium Metal Oxide Nanocrystal on Carbon Textiles. Adv. Energy Mater. 2013, 3, 1484−1489. (13) Liu, B.; Zhang, J.; Wang, X.; Chen, G.; Chen, D.; Zhou, C.; Shen, G. Hierarchical Three-Dimensional ZnCo2O4 Nanowire Arrays/ Carbon Cloth Anodes for a Novel Class of High−Performance Flexible Lithium-Ion Batteries. Nano Lett. 2012, 12, 3005−3011. (14) Hu, L.; Liu, N.; Eskilsson, M.; Zheng, G.; McDonough, J.; Wågberg, L.; Cui, Y. Silicon−Conductive Nanopaper for Li-Ion Batteries. Nano Energy 2013, 2, 138−145. (15) Cheng, Q.; Song, Z.; Ma, T.; Smith, B. B.; Tang, R.; Yu, H.; Jiang, H.; Chan, C. K. Folding Paper−Based Lithium-Ion Batteries for Higher Areal Energy Densities. Nano Lett. 2013, 13, 4969−4974. (16) Song, Z.; Ma, T.; Tang, R.; Cheng, Q.; Wang, X.; Krishnaraju, D.; Panat, R.; Chan, C. K.; Yu, H.; Jiang, H. Origami Lithium−Ion Batteries. Nat. Commun. 2014, 5, 3140. 6233

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