Facile fabrication of highly flexible graphene paper for high-performance flexible lithium ion battery anode

Abstract

Freestanding paper-like materials prepared from chemically derived graphene have considerable potential as a carbon-based electrode in high-performance flexible energy storage devices. Herein, a highly flexible graphene paper (GP) assembled from graphene nanoplatelets (GNPs) with the aid of graphene oxides (GOs) is reported for a high-performance, binder- and conducting additive-free anode in lithium-ion batteries (LIBs). In contrast to previous reports on GPs based on a flow-directed assembly of graphene sheets, this GNP/GO paper exhibited a highly wrinkled and disordered morphology. When the GNP/GO paper was applied as a LIB anode, it showed a high specific capacity of 694 mA h g⁻¹ and high rate performance. Furthermore, a pouch-type flexible LIB using the GNP/GO paper also showed a stable cycling behavior and practical performance. This GNP/GO paper electrode prepared using a simple, yet effective assembly of graphene derivatives, is highly promising for the fabrication of flexible energy storage devices.

---

Introduction

Carbon-based nanostructured materials and their composites are quite intriguing as alternative electrode materials for energy storage/conversion applications because of their definite advantages including lightweight, highly flexible and scalable features.^1–7 Especially graphene, a two-dimensional conducting material comprised of sp²-bonded carbons, has been widely used as a building block for functional composites due to its outstanding electrical and physical properties.^8–11 Attempts to improve the electrochemical performance of these energy storage devices so far has focused on developing various composites based on graphene derivatives as potential anode materials in rechargeable lithium (Li)-ion batteries (LIBs).^12–17 Compared to graphite, graphene has a single-layered structure leading to an intrinsic high surface area for potential Li storage, as Li ion intercalation compounds can form not only on both sides of the graphene sheet, but also on its edges and defect sites.^18,19 In addition, the two-dimensional planar graphene has high mechanical strength and flexibility, enabling the arrangement of individual sheets into freestanding paper-like structures called “graphene paper (GP)” fabricated using a flow-directed assembly.^20,21 As this GP can be used directly as a LIB anode without the addition of binder materials and/or conducting additives, it reduces the electrode's total weight and volume, improving the overall energy density of a device compared to conventional electrodes.

Despite their preparation and utilization advantages, in general, GPs have two major drawbacks. First, GPs fabricated from chemically modified graphene derivatives such as graphene oxide (GO) or reduced graphene oxide (rGO) require an additional thermal annealing process to fully recover the electrical properties of graphene through graphenization of the remaining oxidized carbons.^20,22 This annealing process, which takes place at relatively high temperatures, can destroy the interior structure of the as prepared GPs, making them brittle and fragile under even moderate bending conditions. Second, GPs prepared by vacuum assisted filtration have a well ordered layer structure, resulting from the dense restacking of the individual graphene layers during the filtration process, leading to the film losing several of graphene's merits.^20–24 Theoretically, disordered graphene materials assembled into a “house of cards” architecture show a higher specific capacity (over 780 mA h g⁻¹) compared to ordered graphene materials with a highly crystalline structure such as graphitic carbon (372 mA h g⁻¹).^12,25 As a result, conventional GP electrodes have exhibited poor electrochemical performance compared to composites of graphene powder with a polymer binder, which can be attributed to poor Li ion diffusion and intercalation into the densely packed graphene layers.^23,24 Therefore, an effective control of graphene inter-sheet restacking is critical for enabling the individual graphene sheets in the GP to display their outstanding inherent properties.^26–28

These issues can be remedied by replacing the main GP component with graphene nanoplatelets (GNPs), which are highly conductive few-layer graphene sheets prepared using a chemical exfoliation and thermal reduction of graphite. When using GNPs as a starting material for GP electrode fabrication, no further annealing is required to recover electrical conductivity, which maintains the morphology and structure of the as-prepared GPs and simplifies the fabrication process. Besides, GPs based on GNPs have a disordered and wrinkled structure ascribed to the folding or twisting of the graphene sheets during a previous thermal reduction, which could lead to the formation of a loosely stacked GP structure. As their hydrophobic nature results in poor water dispersion and a strong aggregation during a drying process, nevertheless, GNPs cannot be easily assembled into highly ordered paper-like structures without the aid of relatively large amount of surfactants.

In order to overcome these limitations, we have demonstrated a facile fabrication of highly conductive GPs with wrinkled structures, consisting of GNPs with GOs added as a surfactant and film stabilizer. The main concept of this strategy is shown in Scheme 1. Since the GO sheet is amphiphilic on account of the hydrophobic sp² graphene domains and the hydrophilic oxygen-containing functional groups on the sheet's basal planes and edges, it can be an effective two-dimensional surfactant in an aqueous system.^2,29 As a precursor of chemically derived graphene, GO also has a high specific capacity, sufficient for storing as many Li ions as graphite,³⁰ which makes GO's role not only a structural stabilizer, but also an active Li storage material. This GNP/GO paper displayed a more open, disordered structure with microscopic wrinkles, attributed to a partial GNP aggregation under the stabilizing and holding effects of the GO during the drying process. By virtue of these structural features, the as-prepared GNP/GO paper demonstrated surpassing flexibility and excellent electrochemical performance as a LIB anode. Furthermore, the GNP/GO paper electrode showed great potential for its use in flexible and foldable energy storage devices.

Scheme 1 Schematic illustration demonstrating the concept of the GNP/GO paper. GOs act as a surfactant/stabilizer between GNP sheets, which facilitates the film having a paper-like structure.

Experimental

Preparation of GO and GNP dispersions

GO was synthesized using a modified Hummer's method as described previously.³¹ The resultant GO powder was dispersed in deionized (DI) water using a 20 min bath sonication and centrifuged at 6000 rpm to remove any multi-layered species. A stable GO dispersion with a 0.5 mg mL⁻¹ concentration was prepared from the centrifuged supernatant. The GNP (N002-PDR, X − Y < 10 μm, average thickness < 1 nm) was purchased from Angstron Materials and used as received. The GNP powder was dispersed in DI water at a 0.5 mg mL⁻¹ concentration with a small amount of poly(4-styrenesulfonic acid) (PSS) (Sigma-Aldrich, M_w = 75 000, 18 wt% in water) solution added to help the initial wetting between the GNPs and water (approximately 0.2 mL of PSS solution to 10 mg of GNPs), then the dispersion was stabilized using a horn sonicator for 3 h. Before the GNP/GO paper fabrication, the dispersed GNP and GO solutions were mixed together at a 3 : 1 volume ratio of GNP : GO, followed by adding the same amount of DI water and a 10 min bath sonication for enhanced dispersion.

Preparation of the GNP/GO paper

The GNP/GO papers were obtained using a simple vacuum filtration of the mixed GNP/GO solution through a polyvinylidene fluoride (PVDF) filter (Millipore membranes, Hvlp, 0.45 μm pore size, and 47 mm diameter), followed by drying in an air and separating from the filter membrane. Unless specifically stated, GNP/GO papers with approximately 5 μm thicknesses were used for all experiments, and these were prepared using 20 mL of the mixed GNP/GO solution (0.25 mg mL⁻¹, a total of 5 mg graphene derivatives for a single paper). For the thermally annealed samples, the as-prepared GNP/GO paper was annealed at 350 °C in air for 1 h to remove the oxygen-containing functional groups on GOs and the residual PSS polymer surfactant.

Electrochemical testing

The electrochemical performance of the GNP/GO papers as a LIB anode was investigated using coin-type half cells fabricated in an Argon filled glove box with Li foil as a counter electrode and Celgard 2400 as a separator. The as-prepared GNP/GO paper was punched and dried in a vacuum oven at 120 °C for 1 h to remove residual moistures, then directly used as an electrode without any binder materials and conducting additives. For an electrolyte, 1.0 M LiPF₆ in 1 : 1 v/v ethylene carbonate/dimethyl carbonate (Soulbrain, Korea) was used. All measurements were carried out using a VMP3 potentiostat (BioLogic, France) at room temperature. CV measurements were conducted across a voltage range of 0.01–3.0 V vs. Li/Li⁺ at a 0.1 mV s⁻¹ scan rate, and the galvanostatic charge/discharge tests were conducted across the same voltage range at different current densities.

Characterization

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the raw materials and the GNP/GO paper were obtained using a field-emission scanning electron microscope (Sirion, FEI) and an in situ transmission electron microscope (JEM-3011, JEOL), operating at accelerating voltages of 20 kV and 300 kV, respectively. X-ray diffraction (XRD) patterns were recorded using a high power X-ray diffractometer (D/MAX-2500, Rigaku) with Cu Kα (0.5418 Å) radiation at room temperature. The surface resistances of the GNP/GO papers were measured using the 4 point probe method with a Loresta-GP resistivity meter (MCP-T610, Mitsubishi Chemical). For the conductivity measurements of the GNP/GO paper on bending, a cut and stripe-shaped paper sample was attached to the flexible substrate using Ag paste on both end parts as a glue. Ag paste was also applied to the both end parts of the sample top as an electrode for a probe contact. The resistance of the sample was measured using a HIOKI multi-tester with a direct contact on the Ag electrodes on both sides after repeated bending and folding. Bending and folding of the samples were carried out using a home-made bending machine.

Results and discussion

The flow-directed deposition method by vacuum filtration has been widely used to fabricate paper-like films from a graphene-based colloidal dispersion. Likewise, we prepared GPs with GNPs as the base material using a simple vacuum filtration method. Even with GNP's outstanding electrical properties, it remains difficult to disperse GNP sheets in water and shape a stable paper-like structure after vacuum filtration and drying without the use of additives due to its high hydrophobicity. As water evaporates during the filter drying process, a strong capillary tension is applied to the paper-like sheet as a result of the poor interaction between the GNPs and water. This results in considerable GNP aggregation, generating local shrinkages and overall cracks (left side in Fig. 1a). On the other hand, in the case where GOs are incorporated within the GNPs, the amphiphilic GO sheets can act like a surfactant, infiltrating between the GNP layers to reduce the interfacial tension. The GOs located between the GNP sheets can link to both sheets during a drying process, resulting in an improved stability of the paper-like structure (right side in Fig. 1a). Thus, the as-prepared GNP/GO paper shows high flexibility and even stability when folded or deformed (Fig. 1b and c).

Fig. 1 Optical images of (a) a film obtained using GNP sheets without GO sheets (left) and with GO sheets (right), (b–d) as-prepared GNP/GO paper showing high flexibility and mechanical robustness even under folding deformation.

The microstructure and morphology of the GNP sheet used in this study and the as-prepared GNP/GO paper were characterized by TEM and SEM, as shown in Fig. 2. GNP is a few-layered graphene, as shown in Fig. 2a, consisting of approximately three layers of graphene sheets with a highly wrinkled and crumpled structure, attributed to the chemical exfoliation and thermal reduction of natural graphite.³² These highly wrinkled sheets developed into microscale extrinsic wrinkles and pores through the influence of partial shrinkage and aggregation of the GNPs during the drying process (Fig. 2b). As the added GO sheets hold the paper-like shape, these microscale wrinkles within the individual sheets could prevent dense GNP stacking, leading to a more disordered micro-structure in the resultant paper (Fig. 2c and d). As a result, the GNP/GO paper has a higher degree of disordered structure, with approximately 5 μm thickness, which is much thicker than a paper obtained solely with the same quantity of rGO sheets (approximately 2 μm, Fig. S1 in the ESI†). In addition, the construction of the disordered and porous structure dramatically increased the Brunauer–Emmett–Teller (BET) specific surface area of the resulting paper in comparison to the conventional rGO paper (Table S1 in the ESI†). This implies that the effective assembly of wrinkled GNPs by the aid of GOs facilitates formation of the microscopic wrinkled and porous structure of the final paper without requiring binders and spacers. The microscopic wrinkled and porous structure of the GNP/GO paper could provide efficient diffusion channels for Li ion transport when used as an electrode in LIBs.

Fig. 2 Microstructures and morphologies of a GNP sheet used in this study and as-prepared GNP/GO paper: (a) TEM image of a GNP sheet, (b) top-view and (c and d) cross-sectional-view SEM images of the GNP/GO paper showing highly wrinkled and open structure. Inset in (b) is a corresponding top-view SEM image of the GNP/GO paper with higher magnification.

The microstructure of the as-prepared GNP/GO paper was also investigated using X-ray diffraction (XRD) to analyze the stabilization effect of the GOs, shown in Fig. 3. The paper fabricated only from GOs exhibits a strong single peak (the red line in Fig. 3) centered at 2θ = 11.6°. This peak corresponds to an interlayer distance (d-spacing) of approximately 0.751 nm, indicating that the individual GO sheets stacked in a highly ordered manner during the flow-directed assembly.³³ As the GNP/GO paper consists of two components having different surface structures and morphologies, its XRD pattern was distinct from that of conventional GPs. The peak centered at 2θ = 25.2° was from the interlayer spacing between individual GNP sheets, which is consistent with that of general GPs after they had undergone a thermal reduction process.³⁴ On the other hand, an interesting peak centered at 2θ = 16.7°, which does not appear in general GPs, was observed as a result of the spacing between GO and GNP (approximately 0.532 nm), indicating that the GO sheets are uniformly dispersed and located between the GNP layers in the GNP/GO papers. There may be some few-layered GO sheets in the GO dispersion, although most of the single layer GO sheets were thoroughly separated by centrifugation, thus there was an only a small peak near 2θ = 11.8° corresponding to stacked GOs. This result supports the evidence that the majority of GOs were inserted between the GNP sheets in the resultant GNP/GO paper. Incorporating the GO sheets allows for increased spacing between the stacked GNP layers, providing full utilization of their effective surface areas, thus creating additional storage sites for Li ions.¹⁵ In addition, an unusual peak centered at 2θ = 26.5° was also observed for the GNP/GO paper, which can be attributed to the folded and wrinkled graphene sheet structures.²⁸ Unlike powdered GNPs (Fig. S2 in the ESI†), GNPs after hydration form extrinsic wrinkles that can lead to locally folded and corrugated structures under the applied tensile stress that occurs during a drying process, which is consistent with previous analysis. This structural development, including the expanded spacing and folded structure of the graphene layers in the as-prepared GNP/GO paper, can induce enhanced Li storage capacity when used as a LIB electrode.

Fig. 3 (a) XRD patterns of the GNP/GO paper (black) and a GO paper (red). (b) Schematic illustration of the possible microstructures and the corresponding interlayer spacing in the GNP/GO paper.

The as-prepared GNP/GO paper exhibited outstanding electrical conductivity as well as desirable structural features without any post treatment, which is essential for electrode materials. As the electrical conductivity of the GNP/GO paper can vary based on the paper thickness (Fig. S3 in the ESI†), it reached a maximum of ca. 176 S cm⁻¹ with a thickness of 10 μm. This value did not differ considerably from that of the annealed GNP/GO paper, suggesting that the insulating effect of the nonconductive GOs is negligible. The paper-like structure and relatively high electrical conductivity of the GNP/GO paper enables its direct utilization as an electrode without requiring conducting additive, binder and metal current collector, which can eventually enhance the energy density of a device.

The GNP/GO paper was directly employed as a conducting additive- and binder-free anode for LIBs to investigate its electrochemical performance. As shown in Fig. 4a, the cyclic voltammetry (CV) curves of the GNP/GO paper anode showed a strong reductive peak from 1.0 to 0.24 V for only the first cycle, which could be attributed to the electrochemical reduction of the oxygen-containing functional groups of GO and the formation of a solid-electrolyte interphase (SEI) layer in the porous structure.^35,36 This behavior was supported by the CV curve of a thermally annealed GNP/GO paper anode with no recognizable peak except the Li insertion peak at 1.0 V (Fig. S4 in the ESI†). Fig. 4b shows the voltage profiles for the first four discharge/charge cycles at a constant current density of 50 mA g⁻¹. The potential plateau near 1.0 V was observed for only the first discharge curve, which is consistent with the CV results. The initial discharge capacity reached a maximum 2900 mA h g⁻¹ with an irreversible 76% loss, which could be the result of the electrochemical reduction of GO, the formation of a SEI layer, and/or presumable side reactions of the PSS polymer surfactant. Although the GNP/GO paper anode showed high irreversibility for the first cycle, its irreversible capacity loss sharply decreased with subsequent cycles, indicating that the possible side reactions, including the GO reduction, occurred primarily in the first cycle and finished after a few cycles.

Fig. 4 Electrochemical performances of LIB cells assembled using the GNP/GO paper as anode within 0.01–2.0 V vs. Li/Li⁺. (a) Cyclic voltammetry curves of the GNP/GO paper anode for the first two cycles at a scan rate of 0.1 mV s⁻¹. (b) Galvanostatic discharge/charge potential profiles for the first four cycles at a current density of 50 mA g⁻¹. (c) Specific capacity and coulombic efficiency vs. the cycle number at a current density of 50 mA g⁻¹. (d) Specific capacity and coulombic efficiency vs. the cycle number at different current densities of 100, 200, 300, 500, 1000, and 2000 mA g⁻¹ after the first cycle at 50 mA g⁻¹.

The cycle performance of the GNP/GO paper anode at a current density of 50 mA g⁻¹ is shown in Fig. 4c. The first charge capacity was 694 mA h g⁻¹ with a 24% coulombic efficiency, which was relatively low, but this rapidly increased to over 90% after a few cycles. The charge capacity remained at 530 mA h g⁻¹ after 55 cycles with 76.4% capacity retention. In general, a well-ordered structure of the GP anode presents a kinetic barrier for Li ion diffusion from the anode during lithiation. However, the GNP/GO paper anode, with its disordered and wrinkled structure, provided an increased graphene sheet spacing and more sites for Li ion insertion, resulting in a higher reversible capacity than the densely-stacked conventional GP electrodes prepared using flow-directed methods.^23,24 Fig. 4d shows the GNP/GO paper anode rate performance at current densities of 100, 200, 300, 500, 1000, and 2000 mA g⁻¹, for which the corresponding reversible specific capacities reached 522, 416, 330, 265, 201, and 151 mA h g⁻¹, respectively. As the current density returned to 50 mA g⁻¹, the reversible capacity recovered to 592 mA h g⁻¹, indicating a high rate capability of the GNP/GO paper anode. The GNP/GO paper anode exhibited a high specific capacity and stable electrochemical performance, particularly at high current densities when compared to previously reported GP electrodes.^23,24 This is the result of the GO sheets aiding the formation of a porous GNP stacking, allowing enhanced diffusion and storage of Li ions in its wrinkled and folded structure.

As shown in Fig. 1b and d, the GNP/GO paper was flexible and foldable, allowing its use as an anode in flexible LIBs. In order to examine its potential application in flexible devices, the stability of the GNP/GO paper after bending and folding was investigated. Fig. 5a shows the changes in electrical conductivity after repeated bending and folding, shown as the relative conductance (obtained from the ratio of the initial conductance (G₀) to the measured conductance (G)). The paper was bended to a ca. 38 μm radius of curvature, which is a comparatively smaller bending radius than that of conventional binder-free GPs.³³ The GNP/GO paper maintained not only its paper-like structure but also its initial conductance after 750 bending cycles. In fact, a highly wrinkled paper structure is not particularly appropriate for discussing mechanical strength, because there is a poor load transfer between the overlapped graphene layers.³⁷ Nevertheless, the highly flexible and foldable features of the GNP/GO paper were ascribed to the paper's wrinkled and folded microstructure effectively relieving the applied mechanical stress, which is consistent with what has been found for other wrinkled or buckled structures.^38,39 Even for the case of harsh deformation, such as repeated folding and unfolding, the GNP/GO paper displayed only a small decrease in conductance, within 15%, despite structural fractures seen for the outer layer (Fig. 5b). Maintaining it conductivity under relatively high deformation could be the result of effective sheet linkages by the added GO combined with the extrinsic wrinkled structure of the paper, which may be adjusted further by controlling the size and content of the GO sheets.

Fig. 5 (a) Relative conductance of the GNP/GO paper after repeated bending and folding. (b) Corresponding low-magnification SEM images of bent (up) and folded (down) GNP/GO paper.

GNP/GO paper's highly flexible feature and relatively high conductance even under deformation encouraged us to fabricate a fully flexible LIB using the GNP/GO paper as an anode. Fig. 6a shows the cycle performance of a pouch-type cell charged and discharged under a 15 mm bending radius as shown in Fig. 6b. Except for the first few cycles, the bent cell demonstrated a stable cyclability and decent capacity retention of 66.7% during 55 cycles, suggesting that the GNP/GO paper can be used as a potential electrode for flexible or bendable energy storage devices. The relatively low specific capacities compared with those recorded using the coin-type cell appears to be caused by problems maintaining complete contact among the cell components and difficulties with pouch sealing. In addition, the poor bending stability of the other cell components could be another reason for the relatively low specific capacity of the pouch-type cell with the GNP/GO paper anode. If more flexible cell components are used, along with controlling the cell configuration and improving the cell packing, it is expected that a high-performance flexible LIB using the GNP/GO paper anode can be fabricated.

Fig. 6 (a) Specific capacity vs. cycle number of a flexible pouch-type LIB half-cell using the GNP/GO paper anode at a constant current density of 37.2 mA g⁻¹ (23.8 μA cm⁻²) and (b) optical image for the corresponding demonstration of the bent pouch-type cell with a bending radius of 15 mm. Optical images of (c) flat and (d) bent pouch-type LIB full-cell assembled with a LiCoO₂-coated Al foil as a cathode, showing that a blue LED lights well in both cases after charging to ∼4.0 V.

In addition, we have demonstrated that a pouch-type full LIB cell fabricated with a LiCoO₂-coated Al foil as the cathode and the GNP/GO paper anode can power a blue light emitting diode in both flat and bent configurations after charging to ∼4.0 V. This indicates that the highly flexible and conductive features of the GNP/GO paper anode facilitate its potential for use in flexible and ultra-lightweight energy storage devices as an assembled bulk material. Moreover, this kind of layered structure based on graphene derivatives is a promising candidate as a loading platform for functional nanomaterials (such as metal or metal oxide nanoparticles) with superior electrochemical performance, which can fit into its corrugated structure. Related studies are now progressing as our further works.

Conclusions

We have fabricated a highly flexible and wrinkled GP anode consists of GNPs with the aid of GOs that acted as not only film stabilizers but also active Li storage materials without requiring conducting additives and binders. The as-prepared GNP/GO paper showed increased graphene interlayer spacing and microscopic wrinkles leading to enhanced Li storage and outstanding flexibility. Therefore, the GNP/GO paper anode in LIB exhibited a high specific capacity (694 mA h g⁻¹) and high rate performance compared to previously reported GP electrodes. Furthermore, a pouch-type flexible LIB using the GNP/GO paper as an anode showed a stable and practical performance under even bending deformation, which indicates that this GNP/GO paper electrode can be a promising candidate for high-energy-density and flexible LIBs.

Acknowledgements

This research was supported by a grant (10037689) from the Fundamental R&D Program for Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy, Republic of Korea. D.Y.K and Y.K acknowledge the support from the Government-Funded General Research & Development Program by the Ministry of Trade, Industry and Energy, Republic of Korea.

Notes and references

1. L. Dai, D. W. Chang, J.-B. Baek and W. Lu, Small, 2012, 8, 1130–1166.
2. S. H. Aboutalebi, A. T. Chidembo, M. Salari, K. Konstantinov, D. Wexler, H. K. Liu and S. X. Dou, Energy Environ. Sci., 2011, 4, 1855–1865.
3. J. Liu, L. Zhang, H. B. Wu, J. Lin, Z. Shen and X. W. Lou, Energy Environ. Sci., 2014, 7, 3709–3719.
4. J. Liu, J. Sun and L. Gao, J. Phys. Chem. C, 2010, 114, 19614–19620.
5. H. R. Byon, S. W. Lee, S. Chen, P. T. Hammond and Y. S. Horn, Carbon, 2011, 49, 457–467.
6. J. Liu, C. K. Poh, D. Zhan, L. Lai, S. H. Lim, L. Wang, X. Liu, N. G. Sahoo, C. Li, Z. Shen and J. Lin, Nano Energy, 2013, 2, 377–386.
7. Z.-D. Huang, B. Zhang, S.-W. Oh, Q.-B. Zheng, X.-Y. Lin, N. Yousefi and J.-K. Kim, J. Mater. Chem., 2012, 22, 3591–3599.
8. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv. Mater., 2010, 22, 3906–3924.
9. J. Zhu, D. Yang, Z. Yin, Q. Yan and H. Zhang, Small, 2014, 10, 3480–3498.
10. Y. Sun, Q. Wu and G. Shi, Energy Environ. Sci., 2011, 4, 1113–1132.
11. J. Zhang, F. Zhao, Z. Zhang, N. Chen and L. Qu, Nanoscale, 2013, 5, 3112–3126.
12. M. Liang and L. Zhi, J. Mater. Chem., 2009, 19, 5871–5878.
13. L. Ji, Z. Lin, M. Alcoutlabi and X. Zhang, Energy Environ. Sci., 2011, 4, 2682–2699.
14. S. Han, D. Wu, S. Li, F. Zhang and X. Feng, Small, 2013, 9, 1173–1187.
15. E. Yoo, J. Kim, E. Hosono, H. Zhou, T. Kudo and I. Honma, Nano Lett., 2008, 8, 2277–2282.
16. G. Wang, X. Shen, J. Yao and J. Park, Carbon, 2009, 47, 2049–2053.
17. P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang and H. Wang, Electrochim. Acta, 2010, 55, 3909–3914.
18. J. R. Dahn, T. Zheng, Y. Liu and J. S. Xue, Science, 1995, 270, 590–593.
19. D. Pan, S. Wang, B. Zhao, M. Wu, H. Zhang, Y. Wang and Z. Jiao, Chem. Mater., 2009, 21, 3136–3142.
20. H. Chen, M. B. Müller, K. J. Gilmore, G. G. Wallace and D. Li, Adv. Mater., 2008, 20, 3557–3561.
21. X. Zhou and Z. Liu, Chem. Commun., 2010, 46, 2611–2613.
22. C.-M. Chen, J.-Q. Huang, Q. Zhang, W.-Z. Gong, Q.-H. Yang, M.-Z. Wang and Y.-G. Yang, Carbon, 2012, 50, 659–667.
23. C. Wang, D. Li, C. O. Too and G. G. Wallace, Chem. Mater., 2009, 21, 2604–2606.
24. A. Abouimrane, O. C. Compton, K. Amine and S. T. Nguyen, J. Phys. Chem. C, 2010, 114, 12800–12804.
25. T. Zheng, J. S. Xue and J. R. Dahn, Chem. Mater., 1996, 8, 389–393.
26. X. Yang, J. Zhu, L. Qiu and D. Li, Adv. Mater., 2011, 23, 2833–2838.
27. G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu and J. Lian, Small, 2012, 8, 452–459.
28. F. Liu, S. Song, D. Xue and H. Zhang, Adv. Mater., 2012, 24, 1089–1094.
29. J. Kim, L. J. Cote and J. Huang, Acc. Chem. Res., 2012, 45, 1356–1364.
30. P. Touzain, R. Yazami and J. Maire, J. Power Sources, 1985, 14, 99–104.
31. S. H. Lee, H. W. Kim, J. O. Hwang, W. J. Lee, J. Kwon, C. W. Bielawski, R. S. Ruoff and S. O. Kim, Angew. Chem., Int. Ed., 2010, 49, 10084–10088.
32. G. Srinivas, Y. Zhu, R. Piner, N. Skipper, M. Ellerby and R. Ruoff, Carbon, 2010, 48, 630–635.
33. D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen and R. S. Ruoff, Nature, 2007, 448, 457–460.
34. Z. Hu, Y. Chen, Q. Hou, R. Yin, F. Liu and H. Chen, New J. Chem., 2012, 36, 1373–1377.
35. W. Lu and D. D. L. Chung, Carbon, 2001, 39, 493–496.
36. G. Ning, C. Xu, Y. Cao, X. Zhu, Z. Jiang, Z. Fan, W. Qian, F. Wei and J. Gao, J. Mater. Chem. A, 2013, 1, 408–414.
37. X. Shen, X. Lin, N. Yousefi, J. Jia and J.-K. Kim, Carbon, 2014, 66, 84–92.
38. C. Yu, C. Masarapu, J. Rong, B. Wei and H. Jiang, Adv. Mater., 2009, 21, 4793–4797.
39. J. B. Kim, P. Kim, N. C. Pégard, S. J. Oh, C. R. Kagan, J. W. Fleischer, H. A. Stone and Y. L. Loo, Nat. Photonics, 2012, 6, 327–332.

---

Footnotes

† Electronic supplementary information (ESI) available: Optical and SEM images of a rGO paper, XRD pattern of GNP powder, plots of thickness and conductivity for the GNP/GO paper according to a volume of GNP/GO dispersion, CV curves for a thermally annealed GNP/GO paper, and BET specific surface areas of the papers. See DOI: 10.1039/c4ra13164a

‡ These authors contributed equally to this work.

§ Current address: Energy Lab., Samsung Advanced Institute of Technology (SAIT), Samsung Electronics, Suwon 443-803, Republic of Korea.

---