Fe3O4-decorated hollow graphene balls prepared by spray pyrolysis process for ultrafast and long cycle-life lithium ion batteries

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Fe3O4-decorated hollow graphene balls prepared by spray pyrolysis process for ultrafast and long cycle-life lithium ion batteries Seung Ho Choi a, Yun Chan Kang a b

b,*

Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Fe3O4-decorated graphene balls were prepared by a spray pyrolysis process. Analysis by

Received 18 March 2014

Raman spectroscopy and X-ray photoelectron spectroscopy indicated that the spray pyro-

Accepted 14 July 2014

lysis at 800 C resulted in the complete reduction of graphene oxide sheets containing oxy-

Available online 21 July 2014

gen functional groups into graphene sheets, leading to the formation of Fe3O4-decorated graphene balls. The graphene content in the composite ball was 27 wt%. The Brunauer– Emmett–Teller surface area of the Fe3O4-decorated graphene balls was as high as 130 m2 g 1. The initial discharge and charge capacities of the Fe3O4-decorated graphene balls at a high current density of 7 A g

1

were 1210 and 843 mAh g 1, respectively, and

the discharge capacity was as high as 690 mAh g

1

even after 1000 cycles. The stable revers-

ible discharge capacities of the Fe3O4-decorated graphene balls decreased from 1040 to 540 mAh g

1

with the increase in current density from 1 to 30 A g 1. The Fe3O4-decorated

graphene balls with a uniform distribution of ultrafine Fe3O4 nanocrystals below 15 nm showed superior electrochemical properties as anode materials for lithium ion batteries. The overall structure of the Fe3O4-decorated graphene balls was maintained even after long-term cycling.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

To achieve rechargeable lithium-ion batteries (LIBs) with superior electrochemical properties, electrode materials with novel and functional structures exhibiting higher specific capacities, faster rate performances, longer cycle life, and lower cost are key requirements [1–8]. Transition metal oxides such as FeOx, CoOx, and MnOx, which possess high theoretical specific capacities and are eco-friendly, are potential nextgeneration electrode materials [1–13]. Unfortunately, most transition metal oxides have exhibited a poor cycle life in

* Corresponding author. E-mail address: [email protected] (Y.C. Kang). http://dx.doi.org/10.1016/j.carbon.2014.07.042 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

practical Li+ storage tests because of the volume expansion and low conductivity of the metal oxides [1–13]. Graphenebased composites in which transition metal oxide nanocrystals are uniformly decorated over the graphene substrate have been considered as materials, which can potentially overcome the disadvantages of using transition metal oxides as anode materials in LIBs [14–19]. The graphene-based composites have showed superior electrochemical properties compared to those shown by the bare metal oxides. Graphene sheets, which show excellent electrical conductivity, flexibility, and structural stability, proved to be excellent substrates

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for transition metal oxides [14–20]. Therefore, various morphologies of graphene (or graphene oxide)-metal oxide composites (e.g., anchored, wrapped, sandwich-like, layered, etc.), have been considered as anode materials [14–19]. Graphene-based composite powders have been mainly prepared by liquid solution processes. However, the serious aggregation between the graphene sheets due to van der Waals forces poses serious challenges in the large scale production of sheet-type graphene-based composite powders. Also, the restacking of graphene sheets reduces the impact of the properties of single graphene sheets. Among various metal oxides, Fe3O4 is considered as the most promising anode material because of its high theoretical capacity, low potential, nontoxic nature, and low cost [9,10,21–39]. Graphene–Fe3O4 composites with various configurations including graphene-wrapped, laminated, encapsulated, and core–shell structures have been explored for their potential as anode materials in LIBs [21–36]. Zhou et al. reported graphene-wrapped Fe3O4 anode materials prepared by the in situ reduction of iron hydroxide between graphene nanosheets [21]. Wei et al. proposed a novel strategy to fabricate three-dimensional graphene foams crosslinked with Fe3O4 nanospheres encapsulated with graphene sheets [22]. Recently, crumpled graphene ball prepared by spray pyrolysis using a colloidal solution of graphene oxide have shown excellent compression and aggregation resistance properties [40,41]. Chen et al. reported the nanosized metal oxide or metal-filled graphene nanosacks prepared by a simple aerosol process [42,43]. The crumpled graphene decorated with metal oxide nanoparticles prepared directly by spray pyrolysis have shown promise as anode materials in LIBs [44–47]. The morphologies and electrochemical properties of the crumpled graphene-based materials were strongly affected by the types of metal oxides used to decorate the graphene. However, to the best of our knowledge, the preparation of hollow graphene ball uniformly decorated with ultrafine metal oxide nanocrytals has not been reported for application in LIBs. In this manuscript, we report for the first time a simple continuous method for the production of Fe3O4-decorated graphene balls by scalable spray pyrolysis from a stable graphene oxide colloidal solution containing a Fe salt. The graphene ball with uniform distribution of ultrafine Fe3O4 nanocrystals below 15 nm showed superior electrochemical properties as an anode material for LIBs. The discharge capacity was as high as 690 mAh g 1 after 1000 cycles at a high current density of 7 A g 1. The prepared Fe3O4-decorated graphene balls showed the best ever reported electrochemical properties as anode materials for LIBs.

2.

Experimental section

2.1.

Synthesis of Fe3O4-decorated crumbled graphene balls

The graphene oxide (GO) was synthesized through a modified Hummer’s method [47]. Fe3O4-decorated graphene balls were directly prepared by spray pyrolysis. A schematic of the ultrasonic spray pyrolysis system is shown in Fig. S1. A quartz reactor with a length of 1200 mm and diameter of 50 mm was used for the process. The temperature was maintained at 800 C.

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The nitrogen flow rate (carrier gas) was 10 L min 1. The asobtained GO was redispersed in distilled water and then exfoliated to generate GO sheets by ultrasonication. Then, 500 mL of the exfoliated GO suspension (1 mg ml 1) was added to 4.59 g of iron(III) chloride. Subsequently, the GO with iron chloride salt was dispersed in distilled water by ultrasonication and magnetic stirring. The production rate of the Fe3O4-decorated graphene composite balls was about 1 g h 1.

2.2.

Characterizations

The crystal structures of the powders were investigated by Xray diffractometry (XRD, X’pert PRO MPD) using Cu Ka radiation ˚ ). The morphological features were investigated (k = 1.5418 A using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100F) at a working voltage of 200 kV. The specific surface areas of the Fe3O4-decorated graphene balls were calculated from the Brunauer–Emmett– Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000). The pore size distribution curve was obtained by the Barret–Joyner–Halenda (BJH) method using a TriStar 3000. The XPS spectra of the powders were investigated using X-ray photoelectron spectroscopy (XPS, ESCALAB-250) with Al Ka radiation (1486.6 eV) at the Korea Basic Science Institute (Busan). Thermal gravimetric analysis (TGA; SDT Q600) was performed in air at a heating rate of 10 C min 1 to determine the amount of graphene sheets in the powders.

2.3.

Electrochemical measurements

The capacities and cycling properties of the powders were determined using a 2032-type coin cell. The electrode was prepared from a mixture containing 70 wt% of the active material, 20 wt% of Super P, and 10 wt% of CMC binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was a solution of 1 M LiPF6 in a 1:1 volume mixture of fluoro ethylene carbonate/dimethyl carbonate (FEC/DMC). The charge–discharge characteristics of the samples were determined through cycling in the potential range 0.001–3.0 V at various fixed current densities. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.1 mV s 1. The size of the negative electrode was 1 · 1 cm and the mass loading was about 1.5 mg cm 2.

3.

Results and discussion

Fig. 1 shows the formation mechanism of the Fe3O4-decorated hollow graphene balls by the simple spray pyrolysis. Droplets, which were several microns in size, were formed from the colloidal spray solution of well-dispersed GO and iron chloride using an ultrasonic nebulizer. Fast drying and decomposition of the droplets containing iron chloride and GO occurred in the front portion of the reactor maintained at 800 C. The droplets on drying formed crumpled GO balls of a few microns in size. The decomposition of the FeCl3 under N2 atmosphere resulted in Fe3O4 nanocrystals uniformly dispersed over the crumpled GO. The direct thermal reduction

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of the GO sheets at 800 C resulted in the formation of Fe3O4decorated graphene balls. Hence, the Fe3O4-decorated hollow graphene balls were directly prepared by a simple spray pyrolysis. This unique Fe3O4–graphene composite structure will improve electronic conductivity of pure Fe3O4 material. The void space between graphene sheets could be acted as buffer space for the volume expansion of Fe3O4 during repeatedly Li+ insertion and desertion. The morphologies of the Fe3O4-decorated graphene balls prepared directly by spray pyrolysis at 800 C are shown in Fig. 2. The SEM and TEM images (Fig. 2a and b) showed spherical Fe3O4-decorated graphene balls with wrinkled surfaces. The transparent TEM images (Fig. 2b and c) indicated hollow Fe3O4-decorated graphene balls with empty space. The bent graphene sheets, which were formed during drying as a result of the shrinking of the flat GO sheets present in the droplet, are shown in Fig. 2c. The selected-area electron diffraction (SAED) pattern with a regular and clear ring of diffraction spots array shown in Fig. 2d indicated the polycrystalline structure of Fe3O4. The HR-TEM and dot-mapping image shown in Fig. 2e–g indicated that the ultrafine Fe3O4 nanocrystals were uniformly distributed over the flexible graphene sheets. Fig. 2f shows the HR-TEM image of the Fe3O4 nanocrystals of sizes below 15 nm decorating the graphene sheet. The inset image in Fig. 2f shows the clear lattice fringes separated by 0.29 nm, which corresponds to the (2 2 0) plane of Fe3O4 [23,24]. The dot-mapping images shown in Fig. 2g shows clear voids hidden within the graphene composite ball. Fig. 3a shows the XRD patterns of the Fe3O4-decorated graphene balls prepared directly by spray pyrolysis at

temperatures of 600, 800, and 1000 C. The composite balls prepared by the simple spray pyrolysis process exhibited pure crystal structures of magnetite Fe3O4 (JCPDS card No. 75-0449) irrespective of the preparation temperature. The mean crystallite sizes of the Fe3O4-decorated graphene balls prepared at 600, 800, and 1000 C were 10, 15, and 32 nm, respectively, as calculated using Scherrer’s equation and from the full width at half maximum of the (3 1 1) XRD peaks. The Fe3O4decorated graphene balls exhibited an ultrafine mean crystallite size of 15 nm even at a high preparation temperature of 800 C because the GO and graphene sheets prevented the crystal growth of the Fe components. Fig. S2 presents the Raman spectra of the Fe3O4-decorated graphene balls prepared at various temperatures. The two signals with similar peak intensities in the Raman spectrum of the Fe3O4-decorated graphene balls prepared at 800 C observed at 1350 and 1594 cm 1 corresponded to the D and G graphene bands [23,25]. However, the signal peak intensity of the G band was higher than that of the D band in the Raman spectrum of the Fe3O4-decorated graphene balls prepared at a low temperature of 600 C, in which complete reduction of GO into graphene did not occur. Fig. 3b shows the XPS profile of C1s acquired from the Fe3O4-decorated graphene balls prepared at 800 C. The C1s peak in the XPS profile could be deconvoluted into four components i.e., sp2 bonded carbon (CAC), epoxy and alkoxy groups (CAO), carbonyl and carboxylic (C@O) groups, and COOH group, which corresponded to peaks at 284.6, 286.6, 288.1, and 289.7 eV, respectively [26,48,49]. The XPS profile shown in Fig. 3b showed a sharp peak at 284.6 eV, which could be assigned to graphitic carbon. The relative

Fig. 1 – Schematic diagram of formation and lithium insertion/desertion mechanisms of Fe3O4 decorated hollow graphene ball. (A color version of this figure can be viewed online.)

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Fig. 2 – Morphologies of the Fe3O4-decorated hollow graphene ball powders. (a) SEM image, (b and c) TEM images, (d) SAED pattern, (e and f) HR-TEM images, (g) dot-mapping of Fe, C, and O components. (A color version of this figure can be viewed online.)

carbon content of the sp2 bonded carbon at 286.4 eV was 80% in the Fe3O4-decorated graphene balls. Analysis by Raman spectroscopy and XPS indicated that the complete reduction of GO sheets containing oxygen functional groups into graphene occurred when the preparation temperature was set to 800 C, which resulted in the formation of Fe3O4-decorated graphene balls even though the residence time of the balls within the reactor was as small as 5 s. The typical characteristic peaks corresponding to Fe 2p3/2 and Fe 2p1/2 at 710.9 and 724.5 eV, respectively were observed in the high-resolution XPS profiles acquired from the Fe3O4-decorated graphene balls, as shown in Fig. S3 [22,30]. The graphene content in the Fe3O4-decorated graphene ball was evaluated to be 27 wt% by thermogravimetric analysis (Fig. 3c). Fig. 3d shows the N2 adsorption and desorption isotherms of the Fe3O4-decorated graphene balls prepared by spray pyrolysis at 800 C. The hysteresis loop resembles type-H3 IUPAC (International Union of Pure and Applied Chemistry) isotherm classification, which may have resulted from the slit-shaped pores between the parallel graphene layers [50,51]. The pore size distribution, as shown in the inset of Fig. 3d, demonstrated the

mesoporous structure of the Fe3O4-decorated graphene ball. The BET surface area of the Fe3O4-decorated graphene ball was high as 130 m2 g 1. The electrochemical properties of the Fe3O4-decorated graphene balls were investigated at room temperature (25 C). Fig. 4a shows the cyclic voltammograms (CVs) of the Fe3O4-decorated graphene ball during the first four cycles at a scan rate of 0.1 mV s 1 in the voltage range 0.001–3 V. The first cathodic scan gave rise to two peaks located at 0.9 and 0.6 V, which could be attributed to the formation of the SEI layer caused by the irreversible decomposition of the electrolyte and the reduction of Fe3O4 to Fe0 and Li2O [21–32]. Meanwhile, the two peaks at about 1.6 and 1.8–1.9 V, which could be attributed to the oxidation of Fe0 to Fe2+ and Fe3+, were observed in the anodic process [23,52]. In subsequent cycles, the main cathodic peak was observed at 0.8 V. Fig. 4b shows the charge and discharge curves for the first, second, and 100th cycles at a current density of 2 A g 1. The discharge curves for the first and second cycles showed clear plateaus at about 0.6 and 0.9 V, respectively. The initial discharge and charge capacity of the Fe3O4-decorated graphene balls were

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Fig. 3 – Properties of the Fe3O4-decorated hollow graphene ball powders. (a) XRD patterns, (b) XPS spectra of C1s, (c) TG curves, (d) N2 adsorption–desorption isotherms. (A color version of this figure can be viewed online.)

1374 and 974 mAh g 1 at 2 A g 1, respectively, and the corresponding initial Columbic efficiency was 71%. The cathodic and anodic peaks in the CV curves and the charge and discharge curves from the second cycles overlapped well, indicating the outstanding cycling ability for the insertion and extraction of Li-ions of the Fe3O4-decorated graphene balls. Fig. 4c shows the cycling performance of the Fe3O4-decorated graphene balls at a current density of 2 A g 1 between 0.001 V and 3 V. The Fe3O4-decorated graphene balls delivered a discharge capacity of 981 mAh g 1 after 100 cycles, and the corresponding capacity retention measured from the second cycle was 97%. After 100 cycles, the discharge capacities of the Fe3O4-decorated graphene balls increased gradually and reached 1050 mAh g 1 at the 300th cycle, which could be attributed to the formation of a polymeric gel-like film on the active material as well as electrolyte decomposition [21,22]. The Columbic efficiency of the Fe3O4-decorated graphene balls reached approximately 99% after the third cycle and was maintained at this value in the subsequent cycles. The ultrafast cycling capability of the Fe3O4-decorated graphene balls is shown in Fig. 4d, in which the current densities were increased step wise from 1 to 30 A g 1. For each step, ten cycles were measured to evaluate the rate performance. The stable reversible discharge capacities of the Fe3O4-decorated graphene balls decreased marginally from 1040 to 540 mAh g 1 with an increase in the current density from 1 to 30 A g 1. The Fe3O4-decorated graphene balls exhibited extremely superior rate performances. Also, when the current density returned to 1 A g 1, the reversible discharge capacity recovered to 1020 mAh g 1. Moreover, the Fe3O4-decorated graphene balls exhibited an excellent Columbic efficiency of almost 100% even at ultra-high current densities above 10 A g 1. Fig. S4 shows the charge and discharge profiles of the Fe3O4-decorated graphene balls at the tenth cycle at a current density ranging from 1 to 30 A g 1.

The long-term cycling performance of the Fe3O4-decorated graphene balls at a high current density of 7 A g 1 is shown in Fig. 4e. The initial discharge and charge capacities of the Fe3O4-decorated graphene balls were 1210 and 843 mAh g 1, respectively. In the cycling range 2–180, the reversible discharge capacities deceased from 881 to 690 mAh g 1, whereas the discharge capacities increased to 788 mA h g 1 in the cycling range 180–400. After the 1000th cycle, the Fe3O4-decorated graphene balls showed a high discharge capacity of 690 mAh g 1. The initial decrease in the discharge capacity (2–180 cycles) was related to the transformation of the crystalline structure to a stable amorphous-like structure during cycling. During the high-rate test (7 A g 1), the complete transformation into a stable amorphous-like structure at the discharge state needed cycling for 180 times. Fig. S5 shows the cycling performance of the Fe3O4-decorated graphene balls tested at a current density of 5 A g 1. The 2nd, 180th, and 500th discharge capacities were 957, 897, and 956 mAh g 1, respectively. As shown in Figs. 3c, 4a, and S5, the decreases in the discharge capacities during the initial stages at current densities of 2 and 5 A g 1 were smaller than that obtained at a high current density of 7 A g 1. Similar results have been reported for the high-rate tests for metal oxide electrodes [30,36]. Electrochemical impedance spectroscopy (EIS) was conducted to determine the Li+ transfer behavior in the Fe3O4decorated graphene balls at room temperature before and after the first, 180th, and 500th cycles, in the potential range 0.001–3.0 V at a current density of 7 A g 1. The medium-frequency semicircle was assigned to the charge-transfer resistance (Rct) and the line, which inclined at approximately 45 to the real axis, corresponded to the lithium diffusion process within the electrodes [23,47,53–55]. The diameter of the semicircle obtained before cycling in the medium-frequency region for the Fe3O4-decorated graphene balls was larger than

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Fig. 4 – Electrochemical properties of the Fe3O4-decorated hollow graphene ball powders. (a) CV curves, (b) initial charge/ discharge curves at a constant current density of 2 A g 1, (c) cycle performances at a constant current density of 2 A g 1, (d) rate performances, (e) long cycle performances at an extremely high current density of 7 A g 1. (A color version of this figure can be viewed online.)

that of the semicircle obtained after the first cycle, as shown in Fig. S6. These results represent the transformation of the Fe3O4 nanocrystals into nearly amorphous Fe and Li2O composites [36,56]. The diameter of the medium-frequency semicircle in the EIS spectrum increased after the first cycle with increasing cycle number, and the diameter did not change after 180 cycles even up to 500 cycles, as shown in Fig. S6. Fig. S7 shows the relationship between Zre and x 1/2 (where x is the angular frequency in the low frequency region, x = 2pf) in the low-frequency region, which helps in understanding the cycle performance at a high current density as shown in Fig. 4e. The lithium ion diffusion coefficient (D) is represented by the following equation. D = 0.5 (RT/AF2Cr)2, where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, F is the Faraday constant, C is the concentration of lithium ions, and r is the Warburg impedance coefficient [53–55]. The Warburg impedance coefficient, which is inversely proportional to D, was calculated to be 92, 59, 10, and 7 before and after the before, 1st, 200th, and 500th cycles, respectively for the Fe3O4-decorated graphene balls. The formation of a stable amorphous-like structure after 180 cycles enhanced the Li+ diffusion rate and facilitated rapid Li+ charge/discharge. The morphology changes in the Fe3O4-decorated graphene balls were investigated for the fully charged state after the

500th cycle, as shown in Fig. 5. The overall structure of the Fe3O4-decorated graphene ball was maintained even after long-term cycling, as shown in the TEM images. The HRTEM images (Fig. 5b and c) and SAED pattern (Fig. 5d) with a faint ring diffraction spot show the amorphous-like structure of the Fe3O4-decorated graphene balls after 500 cycles. The Fe component was uniformly dispersed over the graphene balls without aggregation even after cycling, as shown in the dotmapping images. The electrochemical properties of the Fe3O4-decorated graphene balls prepared by the simple spray pyrolysis are compared to those of excellent Fe3O4-graphene composite powders with various morphologies reported previously in the literature and the results are summarized in Table S1. The Fe3O4-graphene composite with graphene content of 20 wt% synthesized by a hydrothermal method exhibited a discharge capacity of 400 mAh g 1 after 500 cycles at a current density of 6 C [30]. The Fe3O4–graphene composite synthesized by a co-precipitation method exhibited a discharge capacity of 460 mAh g 1 after 800 cycles at a current density of 4 A g 1 [31]. The Fe3O4-decorated graphene balls obtained in the present study seem to show markedly superior cycling and rate performances compared to values previously reported for Fe3O4–graphene composite powders. The electrochemical properties of the metal oxide–graphene composite

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Fig. 5 – TEM images, SAED pattern, and dot-mapping images of the Fe3O4-decorated hollow graphene ball after 500th cycle. (A color version of this figure can be viewed online.)

will be strongly affected by the particle size as well as the dispersion uniformity of metal oxide decorated in graphene. The use of water soluble metal salt enabled the formation of Fe3O4–graphene composite balls with uniform distribution of ultrafine Fe3O4 nanoparticles. The crumpled graphene ball backbone with high strength and good flexibility can effectively buffer the structural stress caused by the volume change of iron oxide and prevent the aggregation of Fe3O4 nanoparticles during the lithium ion intercalation/deintercalation cycles. The optimum size of Fe3O4 nanoparticles uniformly decorated in graphene balls also improved the electrochemical properties of the Fe3O4-decorated graphene balls [57]. In the spray pyrolysis, the size of metal oxide dispersed in graphene ball could be controlled by changing the

concentration of metal salt dissolved in the spray solution and the preparation temperature.

4.

Conclusions

In this paper, the morphologies and electrochemical properties of the Fe3O4–graphene composite powders directly prepared by spray pyrolysis were investigated. The direct thermal reduction of GO balls formed during the drying stage of the droplets resulted in the formation of the Fe3O4-decorated hollow graphene balls. The ultrafine Fe3O4 nanocrystals were uniformly distributed over the flexible graphene sheets composing the ball. The Fe3O4-decorated graphene balls exhibited an ultrafine mean crystallite size of 15 nm even at

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a high preparation temperature of 800 C because the GO and graphene sheets prevented the crystal growth of the Fe components. The Fe3O4-decorated graphene balls showed high initial discharge and charge capacities of 1374 and 974 mAh g 1, respectively, at a current density of 2 A g 1 and showed extremely superior cycling and rate performances. The formation of a stable amorphous-like structure during cycling enhanced the Li+ diffusion rate and facilitated fast Li+ charge/discharge process. The structural stability of the Fe3O4-decorated graphene balls during long-term cycling improved the cycling performance at high current densities. The Fe3O4 remained uniformly dispersed over the graphene ball without aggregation even after long-term cycling.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2012R1A2A2A02046367).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2014.07.042.

R E F E R E N C E S

[1] Bruce PG, Scrosati B, Tarascon JM. Nanomaterials for rechargeable lithium batteries. Angew Chem Int Ed 2008;47(16):2930–46. [2] Reddy MV, Rao GVS, Chowdari BVR. Metal oxides and oxysalts as anode materials for Li ion batteries. Chem Rev 2013;113(7). 5364-57. [3] Wu H, Chan G, Choi JW, Ryu I, Yao Y, McDowell MT, Lee SW, Jackson A, Yang Y, Hu L, Cui Y. Stable cycling of double-walled silicon nanotube battery anodes through solid–electrolyte interphase control. Nat Nanotechnol 2012;7:310–5. [4] Wang Z, Zhou L, Lou XW. Metal oxide hollow nanostructures for lithium-ion batteries. Adv Mater 2012;24(14):1903–11. [5] Scrosati B, Hassoun J, Sun YK. Lithium-ion batteries. A look into the future. Energy Environ Sci 2011;4(9):3287–95. [6] Lai XY, Halpert JE, Wang D. Recent advances in micro-/nanostructured hollow spheres for energy applications: from simple to complex systems. Energy Environ Sci 2012;5(2):5604–18. [7] Zhang HL, Zhang Y, Zhang XG, Li F, Liu C, Tan J, et al. Urchinlike nano/micro hybrid anode materials for lithium ion battery. Carbon 2006;44(13):2778–84. [8] Choi NS, Chen ZH, Freunberger SA, Ji XL, Sun YK, Amine K, et al. Challenges facing lithium batteries and electrical double-layer capacitors. Angew Chem Int Ed 2012;51(40):9994–10024. [9] Zhang L, Wu HB, Lou XW. Iron-oxide-based advanced anode materials for lithium-ion batteries. Adv Energy Mater 2014;4(4):1300958. [10] Lee SH, Yu SH, Lee JE, Jin AH, Lee DJ, Lee NH, et al. Selfassembled Fe3O4 nanoparticle clusters as high-performance anodes for lithium ion batteries via geometric confinement. Nano Lett 2013;13(9):4249–56.

65

[11] Wang H, Cui LF, Yang Y, Casalongue HS, Robinson JT, Liang Y, et al. Mn3O4–graphene hybrid as a high-capacity anode material for lithium ion batteries. J Am Chem Soc 2010;132(40):13978–80. [12] Li Y, Zhu C, Lu T, Guo Z, Zhang D, Ma J, et al. Simple fabrication of a Fe2O3/carbon composite for use in a highperformance lithium ion battery. Carbon 2013;52:565–73. [13] Su DW, Ford M, Wang GX. Mesoporous NiO crystals with dominantly exposed 1 1 0 reactive facets for ultrafast lithium storage. Sci Rep 2012;2:924. [14] Wu ZS, Zhou GM, Yin LC, Ren WC, Li F, Cheng HM. Graphene/ metal oxide composite electrode materials for energy storage. Nano Energy 2012;1(1):107–31. [15] Sun YQ, Wu Q, Shi GQ. Graphene based new energy materials. Energy Environ Sci 2011;4(4):1113–32. [16] Choi HJ, Jung SM, Seo JM, Chang DW, Dai LM, Baek JB. Graphene for energy conversion and storage in fuel cells and supercapacitors. Nano Energy 2012;1(4):534–51. [17] Huang X, Qi XY, Boey F, Zhang H. Graphene-based composites. Chem Soc Rev 2012;41(2):666–86. [18] Xin S, Guo YG, Wan LJ. Nanocarbon networks for advanced rechargeable lithium batteries. Acc Chem Res 2012;45(10):1759–69. [19] Gong YJ, Yang SB, Liu Z, Ma LL, Vajtai R, Ajayan PM. Graphene-network-backboned architectures for highperformance lithium storage. Adv Mater 2013;25(29):3979–84. [20] Zhu YW, Murali ST, Cai WW, Li XS, Suk JW, Potts JR, et al. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22(35):3906–24. [21] Zhou GM, Wang DW, Li F, Zhang LL, Li N, Wu ZS, et al. Graphene-wrapped Fe3O4 anode material with improved reversible capacity and cyclic stability for lithium ion batteries. Chem Mater 2010;22(18):5306–13. [22] Wei W, Yang SB, Zhou HX, Lieberwirth I, Feng XL, Mu¨llen K. 3D graphene foams cross-linked with pre-encapsulated Fe3O4 nanospheres for enhanced lithium storage. Adv Mater 2013;25(21):2909–14. [23] Wang JZ, Zhong C, Wexler D, Idris NH, Wang ZX, Chen LQ, et al. Graphene-encapsulated Fe3O4 nanoparticles with 3D laminated structure as superior anode in lithium ion batteries. Chem Eur J 2011;17(2):661–7. [24] Zhuo LH, Wu YQ, Wang LY, Ming J, Yu YC, Zhang XB, et al. CO2-expanded ethanol chemical synthesis of a Fe3O4@graphene composite and its good electrochemical properties as anode material for Li-ion batteries. J Mater Chem A 2013;1(12):3954–60. [25] Chen DZ, Quan HY, Liang JF, Guo L. One-pot synthesis of hematite@graphene core@shell nanostructures for superior lithium storage. Nanoscale 2013;5(20):9684–9. [26] Kim IT, Magasinski A, Jacob K, Yushin G, Tannenbaum R. Synthesis and electrochemical performance of reduced graphene oxide/maghemite composite anode for lithium ion batteries. Carbon 2013;52:56–64. [27] Zhang M, Jia MQ, Jin YH. Appl Surf Sci 2012;261:298–305. [28] Lian PC, Zhu XF, Xiang HF, Li Z, Yang WS, Wang HH. Enhanced cycling performance of Fe3O4–graphene nanocomposite as an anode material for lithium-ion batteries. Electrochim Acta 2010;56(2):834–40. [29] Yang SB, Sun Y, Chen L, Hernandez YN, Feng XL, Mu¨llen K. Porous iron oxide ribbons grown on graphene for highperformance lithium storage. Sci Rep 2012;2:427. [30] Luo JS, Liu JL, Zeng ZY, Ng CF, Ma LJ, Zhang H, et al. Threedimensional graphene foam supported Fe3O4 lithium battery anodes with long cycle life and high rate capability. Nano Lett 2013;13(12):6136–43. [31] Behera SK. Enhanced rate performance and cyclic stability of Fe3O4–graphene nanocomposites for Li ion battery anodes. Chem Commun 2011;47(37):10371–3.

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[32] Zhou Q, Zhao Z, Wang Z, Dong Y, Wang X, Gogotsi Y, et al. Low temperature plasma synthesis of mesoporous Fe3O4 nanorods grafted on reduced graphene oxide for high performance lithium storage. Nanoscale 2014;6(4):2286–91. [33] Chen Y, Song BH, Li M, Lu L, Xue JM. Ultra-small Fe3O4 nanoparticle decorated graphene nanosheets with superior cyclic performance and rate capability. Nanoscale 2013;5(15):6797–803. [34] Zhu XJ, Zhu YW, Murali ST, Stoller MD, Ruoff RS. Nanostructured reduced graphene oxide/Fe2O3 composite as a high-performance anode material for lithium ion batteries. ACS Nano 2011;5(4):3333–8. [35] Zou YQ, Kan J, Wang Y. Fe2O3–graphene rice-on-sheet nanocomposite for high and fast lithium ion storage. J Phys Chem C 2011;115(42):20747–53. [36] Lin J, Raji AO, Nan K, Peng Z, Yan Z, Samuel ELG, et al. Iron oxide nanoparticle and graphene nanoribbon composite as an anode material for high-performance Li-ion batteries. Adv Funct Mater 2014;24(14):2044–8. [37] Wang B, Wu HB, Zhang L, Lou XW. Self-supported construction of uniform Fe3O4 hollow microspheres from nanoplate building blocks. Angew Chem Int Ed 2013;52(15):4165–8. [38] Chen JS, Zhang Y, Lou XW. One-pot synthesis of uniform Fe3O4 nanospheres with carbon matrix support for improved lithium storage capabilities. ACS Appl Mater Interfaces 2011;3(9):3276–9. [39] Gao G, Yu L, Wu HB, Lou XW. Hierarchical tubular structures constructed by carbon-coated a-Fe2O3 nanorods for highly reversible lithium storage. Small 2014;10(9):1741–5. [40] Luo JY, Jang HD, Sun T, Xiao L, He Z, Katsoulidis AP, et al. Compression and aggregation-resistant particles of crumpled soft sheets. ACS Nano 2011;5(11):8943–9. [41] Luo JY, Jang HD, Huang JX. Effect of sheet morphology on the scalability of graphene-based ultracapacitors. ACS Nano 2013;7(2):1464–71. [42] Chen Y, Guo F, Jachak A, Kim SP, Datta D, Liu J, et al. Aerosol synthesis of cargo-filled graphene nanosacks. Nano Lett 2012;12(4):1996–2002. [43] Chen Y, Guo F, Qiu Y, Hu H, Kulaots I, Walsh E, et al. Encapsulation of particle ensembles in graphene nanosacks as a new route to multifunctional materials. ACS Nano 2013;7(5):3744–53. [44] Mao S, Wen ZH, Kim HJ, Lu GH, Hurley P, Chen JH. A general approach to one-pot fabrication of crumpled graphene-based nanohybrids for energy applications. ACS Nano 2012;6(8):7505–13. [45] Luo JY, Zhao X, Wu JS, Jang HD, Kung HH, Huang JX. Crumpled graphene-encapsulated Si nanoparticles for lithium ion battery anodes. J Phys Chem Lett 2012;3(13):1824–9.

[46] Wen Y, Zhu YJ, Langrock A, Manivannan A, Ehrman SH, Wang CS. Graphene-bonded and -encapsulated Si nanoparticles for lithium ion battery anodes. Small 2013;26(16):2810–6. [47] Choi SH, Kang YC. Crumpled graphene–molybdenum oxide composite powders: preparation and application in lithiumion batteries. ChemSusChem 2014;7(2):523–8. [48] Beidaghi M, Wang C. Micro-supercapacitors based on interdigital electrodes of reduced graphene oxide and carbon nanotube composites with ultrahigh power handling performance. Adv Funct Mater 2012;22(21):4501–10. [49] Song LH, Lim SN, Kang KK, Park SB. Graphene-based mesoporous nanocomposites of spherical shape with a 2-D layered structure. J Mater Chem A 2013;1(23):6719–22. [50] Paek SM, Yoo EJ, Honma I. Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure. Nano Lett 2009;9(1):72–5. [51] Lian PC, Zhu XF, Liang SZ, Li Z, Yang WS, Wang HH. Large reversible capacity of high quality graphene sheets as an anode material for lithium-ion batteries. Electrochim Acta 2010;55(12):3909–14. [52] He Y, Huang L, Cai JS, Zheng XM, Sun SG. Structure and electrochemical performance of nanostructured Fe3O4/ carbon nanotube composites as anodes for lithium ion batteries. Electrochim Acta 2010;55(3):1140–4. [53] Ko YN, Park SB, Jung KY, Kang YC. One-pot facile synthesis of ant-cave-structured metal oxide–carbon microballs by continuous process for use as anode materials in Li-ion batteries. Nano Lett 2013;13(11):5462–6. [54] Shi Y, Wang JZ, Chou SL, Wexler D, Li HJ, Ozawa K, et al. Hollow structured Li3VO4 wrapped with graphene nanosheets in situ prepared by a one-pot template-free method as an anode for lithium-ion batteries. Nano Lett 2013;13(10):4715–20. [55] Ko YN, Park SB, Kang YC. Design and fabrication of new nanostructured SnO2–carbon composite microspheres for fast and stable lithium storage performance. Small 2014. http://dx.doi.org/10.1002/smll.201400613. [56] Su QM, Xie D, Zhang J, Du GH, Xu BS. In situ transmission electron microscopy observation of the conversion mechanism of Fe2O3/graphene anode during lithiation– eelithiation processes. ACS Nano 2013;7(10):9115–21. [57] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nanosized transition-metal oxides as negative-electrode materials for lithium-ion batteries. Nature 2000;407:496–9.