Highly Efficient High-Pressure Homogenization Approach for Scalable

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Highly Efficient High-Pressure Homogenization Approach for Scalable Production of High-Quality Graphene Sheets and SandwichStructured α‑Fe2O3/Graphene Hybrids for High-Performance Lithium-Ion Batteries Xin Qi,† Hao-Bin Zhang,*,† Jiantie Xu,‡ Xinyu Wu,† Dongzhi Yang,† Jin Qu,† and Zhong-Zhen Yu*,† †

Beijing Key Laboratory of Advanced Functional Polymer Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2500, Australia S Supporting Information *

ABSTRACT: A highly efficient and continuous high-pressure homogenization (HPH) approach is developed for scalable production of graphene sheets and sandwich-structured αFe2O3/graphene hybrids by liquid-phase exfoliation of stage-1 FeCl3-based graphite intercalation compounds (GICs). The enlarged interlayer spacing of FeCl3-GICs facilitates their efficient exfoliation to produce high-quality graphene sheets. Moreover, sandwich-structured α-Fe2O3/few-layer graphene (FLG) hybrids are readily fabricated by thermally annealing the FeCl3 intercalated FLG sheets. As an anode material of Li-ion battery, α-Fe2O3/FLG hybrid shows a satisfactory long-term cycling performance with an excellent specific capacity of 1100.5 mA h g−1 after 350 cycles at 200 mA g−1. A high reversible capacity of 658.5 mA h g−1 is achieved after 200 cycles at 1 A g−1 and maintained without notable decay. The satisfactory cycling stability and the outstanding capability of α-Fe2O3/FLG hybrid are attributed to its unique sandwiched structure consisting of highly conducting FLG sheets and covalently anchored α-Fe2O3 particles. Therefore, the highly efficient and scalable preparation of high-quality graphene sheets along with the excellent electrochemical properties of α-Fe2O3/FLG hybrids makes the HPH approach promising for producing high-performance graphene-based energy storage materials. KEYWORDS: high-quality graphene, graphene hybrids, liquid-phase exfoliation, high-pressure homogenization, lithium-ion batteries

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graphene production.14 It is still imperative to develop a continuous and highly efficient method for scalable preparation of high-quality graphene sheets and their derivatives. Graphene has been widely used to prepare anode materials for lithium-ion batteries (LIBs) with remarkable rate capability and high energy density because of its excellent electrical conductivity, high specific surface area, and outstanding structural stability.2,15 Graphene is also hybridized with other types of anode materials to enhance their electrochemical performances and to take advantage of the synergistic effects between the different components.16 Since 2000, a large number of transition metal oxides have been developed as promising anode candidates for LIBs due to their capacities being higher than that of graphite (∼372 mA h g−1).17 Costeffective and eco-friendly Fe2O3 exhibits an excellent theoretical specific capacity of ∼1007 mA h g−1,18−25 and its volume expansion during the lithiation/delithiation processes could be

INTRODUCTION Tremendous attention has been paid to graphene because of its excellent electrical, thermal, and mechanical properties.1−4 To exploit its intriguing potential applications, various methods have been developed for production of graphene sheets, including bottom-up synthesis from organic compounds,5 mechanical cleavage,1,6 chemical reduction of graphene oxide (GO),7,8 chemical vapor deposition (CVD),9 and electrochemical exfoliation.10 Despite these encouraging progresses, the scalable production of high-quality graphene sheets with graphite oxide as their precursor still remains challenging. Liquid-phase exfoliation of pristine graphite flakes by sonication,11 high-shear mixing,12 and high-pressure microfluidization13 has been attempted to prepare high-quality graphene sheets. However, the main drawback is their low efficiency that derives from the constraint of van der Waals forces among adjacent graphene layers. Thus, weakening the adverse restriction by enlarging the interlayer spacing of graphite would facilitate its exfoliation. In our previous work, FeCl3-based graphite intercalation compounds (FeCl3-GICs) were confirmed to be an ideal precursor for high-quality © 2017 American Chemical Society

Received: January 17, 2017 Accepted: March 6, 2017 Published: March 6, 2017 11025

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of the internal structure of HPH and (b) its mechanisms for production of graphene sheets by exfoliating FeCl3-GICs. Preparation and Exfoliation of FeCl3-GICs. Graphite and FeCl3 (1/3, w/w) were mixed in a glovebox filled with argon gas and kept in a 50 mL autoclave. The mixture was heated to 400 °C for 12 h in a muffle furnace to obtain stage-1 FeCl3-GICs. FeCl3-GICs were dispersed in NMP with contents of 0.2 and 1 mg mL−1, and the suspensions were homogenized repeatedly for 5−50 cycles under various pressures between 60 and 150 MPa by using an APV 2000 laboratory high-pressure homogenizer (Germany) with a maximum processing capacity of 11 L h−1. The homogenized suspensions were filtrated, washed with ethanol and water three times, and then transferred for freeze-drying in a Boyikang FD-1C-50 freeze-drier (China) and vacuum-drying to obtain graphene sheets and FeCl3-FLG hybrid. Synthesis of α-Fe2O3/FLG Hybrids. The FeCl3-FLG sheets were put in a corundum boat and heated to 500 °C for 1 h under flowing air to obtain α-Fe2O3/FLG powders, which were designated as α-Fe2O3/ FLG-x, where x was its HPH cycling number. For comparison, FeCl3GICs were heated under the same conditions to get α-Fe2O3/GICs hybrids. Characterization. The microstructures of graphite and its derivatives were characterized by a Renishaw inVia Raman microscopy (514 nm, U.K.) and a Bruker X-ray diffractometer (XRD, Germany). Elemental compositions of the specimens were measured by a Thermo VG RSCAKAB X-ray photoelectron spectroscopy (XPS, U.K.). The morphologies of the specimens were characterized using a JEOL Tecnai G2 20 transmission electron microscope (TEM, Japan), a Hitachi S-4800 scanning electron microscope (SEM, Japan), and a Bruker atomic force microscope (AFM, Germany) in tapping mode. Shimazu DTG-60A thermogravimetric analysis (TGA, Japan) was used to determine α-Fe2O3 content with a heating rate of 10 °C/min from 30 to 900 °C in air. Electrochemical performances were tested using CR2032 half-coin cells. The working electrodes were prepared by casting the mixture of poly(vinylidenefluoride), carbon black, and active materials (1/2/7, w/w/w) on a Ni foam followed by drying at 80 °C for 24 h. The mass loading of a circular electrode (about 1 cm in diameter) was controlled at 1−1.5 mg cm−2. In a high-purity argonfilled glovebox, the cells were assembled with the as-prepared electrode, the lithium foil as the counter electrode, 1 M LiPF6 in ethylene carbonate, dimethyl carbonate and diethyl carbonate (1/1/1, v/v/v) as the electrolyte (Hefei Kejing Materials Technology, China), and glass fiber fabric as the separator membrane. The cycling performances of the electrodes were characterized with a LANDCT2001A electrochemical testing system from 3.00 to 0.01 V at various current densities. Cyclic voltammetry (CV) was carried out with a CHI 660E electrochemical workstation from 3.00 to 0.01 V at 0.1 mV s−1 (vs Li+/Li).

alleviated by combining with carbon materials with rational structures.23−26 With a two-step synthesis process consisting of homogeneous precipitation and subsequent chemical reduction of GO under microwave irradiation, Ruoff et al.27 prepared a Fe2O3/reduced graphene oxide (RGO) nanocomposite which exhibited a good cycling performance and high capacities. Guo et al.28 fabricated a RGO/Fe3O4/Fe2O3 composite by a hydrothermal reaction, showing a high capacity retention of 868.4 mA h g−1 after 300 cycles at 500 mA g−1 and an excellent rate capability of 585.8 mA h g−1 at 2000 mA g−1. Yooh et al.29 synthesized a hollow α-Fe2O3 nanobarrel/RGO nanocomposite with a microwave-assisted hydrothermal method, exhibiting a superior capacity of 916 mA h g−1 at 500 mA g−1 after 100 cycles. Shen et al.30 reported porous Fe2O3 nanorods anchored on 3D carbon nanotube-graphene foams by a hydrothermal method, with a reversible capacity more than 1000 mA h g−1 up to 300 cycles at 200 mA g−1 without obvious fading. To avoid the complicated and time-consuming hybridization processes including preparation of GO, precursor deposition on a substrate, reduction, and annealing treatments,22−26 it is still imperative to explore an efficient approach for mass preparation of Fe2O3/graphene hybrids as high-performance anode materials. Herein, we utilize a highly efficient and continuous highpressure homogenization (HPH) approach for scalable production of graphene and sandwich-structured α-Fe2O3/ few-layer graphene (FLG) hybrids with stage-1 FeCl3-GICs as the precursor. Due to the intercalation of FeCl3, the enlarged interlayer spacing of the GICs ensures the efficient exfoliation of graphene sheets by greatly weakening the constraint of the van der Waals forces between adjacent sheets while retaining their sp2-conjugated structure.14 The unique exfoliation mechanisms of the HPH approach include shearing, crashing, and cavitation,13,31,32 afford the efficient production of highquality graphene sheets with an average thickness of ∼0.72 nm for 50 cycles under 120 MPa. In addition, by controlling parameters of the high-pressure homogenization of FeCl3intercalated few-layer graphene sheets (FeCl3-FLG) and subsequent high-temperature annealing, α-Fe2O3/FLG hybrids are prepared and show a high capacity retention of 1100.5 mA h g−1 after 350 cycles at 200 mA g−1 without apparent decay.

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RESULTS AND DISCUSSION Preparation of High-Quality Graphene Sheets by High-Pressure Homogenization of FeCl3-GICs. The internal structure of the HPH equipment and its mechanisms for the production of graphene sheets are illustrated in Figure 1.

EXPERIMENTAL SECTION

Materials. Natural graphite (300 mesh, 99.9%) was purchased from Huadong Graphite Factory (China), FeCl3 was bought from Aladdin (China), and N-methyl-2-pyrrolidone (NMP, 99.5%) and ethanol were provided by Beijing Chemical Factory (China). 11026

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) XRD and (b) Raman spectra of stage-1 FeCl3-GICs, graphene, and graphite. SEM images of (c) graphite flakes and (d) stage-1 FeCl3GICs. (e, f) TEM images of graphene sheets. The inset of panel e is the NMP suspension of graphene sheets.

Generally, NMP suspension is first delivered to the cavity inside the valve seat and then rapidly accelerated as flowing through the narrow slits between the valve and the valve seat. The generated strong shear force and the violent collision of the suspension on the impact ring facilitate the exfoliation of FeCl3GICs. The created cavitation within microseconds induces a shock wave, breaking the large flakes to pieces. Compared to other techniques of sonication11 and high-shear mixing,12 the HPH approach has two obvious advantages: One is that the high-pressure homogenization process combines well with the shearing, crashing, and cavitation mechanisms, making it highly efficient in producing graphene and its hybrids, and the other is its being a continuous and industrial-compatible process enabling the mass preparation of graphene nanomaterials. The advantage of choosing stage-1 FeCl3-GICs as the precursor of graphene is that the enlarged interlayer spacing greatly reduces the van der Waals force among the adjacent graphene sheets, facilitating their splitting and exfoliation.14,33,34 As shown in Figure 2a, different from the typical peak at 26.4° (002) of pristine graphite with a d-spacing of 0.34 nm,35 the new diffraction peaks at 9.2° (001), 18.6° (002), and 28.1° (003) confirm the formation of stage-1 FeCl3-GICs with an enlarged intragallery of 0.96 nm.14,33,34 Meanwhile, there are no characteristic peaks of Fe2O3 or Fe3O4 in the XRD pattern of

FeCl3-GICs, confirming the intercalant is FeCl3 rather than Fe2O3 or Fe3O4.14,25,28 After 20 cycles of treatment by HPH, the sheets exhibit weak peaks due to the disrupted structure of FeCl3-GICs. Figure 2b shows the Raman spectra of graphite and its derivatives. Upon the intercalation of FeCl3, the G peak of graphite shifts from 1580 to 1621 cm−1 due to the doping effect, again confirming the formation of stage-1 FeCl3GICs.36−38 After the exfoliation, however, the G peak returns to 1580 cm−1, suggesting the removal of the intercalated FeCl3.33 The low intensity ratio of D to G bands (ID/IG < 0.1) along with the weak D band suggests the low-defect and highquality feature of the resulting graphene sheets.39 The layered structures of graphite and stage-1 FeCl3-GICs are confirmed by SEM images (Figure 2c,d). After the intercalation of FeCl3, the regular graphite structure becomes loose. The TEM images reveal that the graphene sheets are ultrathin and transparent with regular fringes (Figure 2e,f). It is seen from the edge that the graphene contains only two individual layers (Figure 2f). It should be mentioned that large amounts (i.e., 11 L h−1) of NMP suspensions containing graphene sheets could be rapidly obtained by the continuous HPH process (inset of Figure 2e). The graphene film has an excellent electrical conductivity of 3.7 × 104 S m−1, which can be used for electrical conductor of LED (Figure S1). 11027

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

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ACS Applied Materials & Interfaces

Figure 3. AFM images of the graphene sheets prepared with HPH under 120 MPa (a) for 20 cycles and (b) their thickness histogram, as well as those (c) for 50 cycles and (d) their thickness histogram. XRD patterns of the graphene sheets prepared (e) under different homogenization pressures for 20 cycles (0.2 mg mL−1) and (f) under 120 MPa for different cycles (1 mg mL−1).

thickness in their AFM image and a sharp characteristic peak at 26.4° in their XRD pattern (Figure S2), further confirming the advantages of FeCl3-GICs as the precursor of graphene sheets. Moreover, the effect of cycling number is also studied under 120 MPa with an initial FeCl3-GICs content of 1 mg mL−1. The typical peaks of GICs are greatly weakened after homogenizing for 5 and 10 cycles (Figure 3f), implying the exfoliation of the bulky GICs flakes to few-layer graphene sheets. When the cycling number is more than 20, these characteristic peaks disappear, indicating the complete exfoliation of GICs and the removal of FeCl3. This result is supported by the returning of the G band from 1621 to 1580 cm−1 for the exfoliated sheets under 120 MPa for 20 cycles (Figure S3). Otherwise, the G band would shift to a higher wavenumber.14 The thicknesses of the graphene sheets at different cycles are determined to be 4.5 nm (5 cycles), 2.5 nm (10 cycles), and 1.1 nm (20 cycles). It is apparent that a large cycling number is beneficial for the preparation of thin graphene sheets. The above-mentioned

The parameters of the HPH process, such as cycle number and pressure, play an important role in determining the exfoliation extent. At pressures greater than 120 MPa, the characteristic peaks of GICs almost disappear, indicating the disruption of their ordered structure. Figure 3a,c compares the graphene sheets prepared under pressures of 120 MPa for 20 and 50 cycles. The thickness of the graphene sheets exfoliated under 120 MPa is 2.25 nm after 20 cycles (Figure 3a) and decreases to 0.67 nm after 50 cycles (Figure 3c). Statistical results from the AFM images reveal that the average thickness is 2.4 nm for the former and becomes 0.72 nm for the latter (Figure 3b,d), indicating a significant reduction in the thickness of graphene sheets with increasing the HPH cycling number. The influence of pressure is reflected on the XRD patterns of FeCl3-GICs and their exfoliated graphene sheets (Figure 3e). High pressure facilitates the exfoliation of FeCl3-GICs to graphene sheets, evidenced by the weakened peaks of GICs. For comparison, the graphene sheets directly exfoliated from pristine graphite under 120 MPa for 20 cycles have a large 11028

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a−d) SEM and (e, f) TEM images of the α-Fe2O3/FLG-5 hybrid. The inset of panel f is a HRTEM image of α-Fe2O3 particles.

Compared to α-Fe2O3/FLG-5 hybrid, α-Fe2O3/FLG-10 hybrid is composed of thinner FLG sheets and less content α-Fe2O3 (Figure S4). Even so, uniformly anchored and sandwiched α-Fe2O3 particles are still observed in the αFe2O3/FLG-10 hybrid. Their TGA curves reveal that the contents of α-Fe2O3 are 48.3 wt % in α-Fe2O3/FLG-5 and 42.0 wt % in α-Fe2O3/FLG-10 hybrid (Figure S5). However, directly annealed FeCl3-GICs lead to a bulky α-Fe2O3/GICs hybrid with densely anchored large α-Fe2O3 particles on both the surfaces and the edges of graphite layers (Figure S6). The formation of α-Fe2O3/FLG hybrids is verified by the XRD peaks of 24.2, 33.2, 35.5, and 40.9° (Figure 5a), which are respectively indexed to (012), (104), (110), and (113) crystal planes of α-Fe2O3 (PDF No. 33−0664, JCPDS, 2004).25 The XRD patterns prove the crystalline form of α-Fe2O3 particles that are anchored on FLG sheets. The peak at 26.4° can be assigned to the (002) plane of the FLG sheets. Raman spectra of graphite, FeCl3-GICs, and α-Fe2O3/FLG hybrids are used to monitor the structural evolution during

results confirm the high efficiency of the HPH approach for scalable preparation of high-quality graphene sheets. Preparation of α-Fe2O3/FLG Hybrids. By controlling the parameters of the HPH approach and subsequent hightemperature annealing, α-Fe2O3/FLG hybrids are readily prepared and their microstructures characterized using SEM and TEM (Figures 4 and S4). After the HPH treatment of FeCl3-GICs only for 5 cycles under 120 MPa, the resultant αFe2O3/FLG-5 hybrid has a much smaller thickness than that of FeCl3-GICs. The α-Fe2O3 particles with an average diameter of ∼180 nm, calculated on the basis of statistic results, not only anchor on the surfaces of FLG but also locate in the space between adjacent graphene sheets (Figure 4a−d). The uniform distribution of α-Fe2O3 on FLG surfaces is also verified with TEM images (Figure 4e,f). The crystalline form of α-Fe2O3 is well-certified by its lattice fringes (Figure 4f) with spacing of 0.18 nm (024) and 0.17 nm (116) (Powder Diffraction File (PDF) No. 33−0664, Joint Committee on Powder Diffraction Standards (JCPDS), 2004).25 11029

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

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ACS Applied Materials & Interfaces

Figure 5. (a) XRD patterns of α-Fe2O3 and α-Fe2O3/FLG hybrids; (b) Raman spectra of graphite, FeCl3-GICs, and α-Fe2O3/FLG hybrids; (c) survey scan, (d) Fe 2p, (e) O 1s, and (f) C 1s of XPS spectra of the α-Fe2O3/FLG-5 hybrid.

the linkage of anchored Fe2O3 on FLG (Figure 5d).25 The O 1s XPS results further verify the formation of Fe−O (530.2 eV), C−O−Fe (531.1 eV), and Fe−OH (532.2 eV) bonds (Figure 5e).41,42 The oxygen peak in C 1s spectrum of α-Fe2O3/FLG-5 is mainly from the α-Fe2O3 component and the oxygen functionalities on FLG sheets formed during the intercalation, exfoliation, and annealing processes (Figure 5f).29,43,44 It is expected that the α-Fe2O3/FLG hybrids would benefit the performance of LIBs by facilitating the charge transfer and intercalation/deintercalation of lithium ions. Electrochemical Properties of α-Fe2O3/FLG Hybrids in LIBs. As shown in Figure 6, the α-Fe2O3/FLG hybrids prepared by the HPH approach exhibit excellent electrochemical properties as electrode materials. Figure 6a presents the CV curves of the α-Fe2O3/FLG-5 electrode. During the first cathodic scan, the peak at 0.75 V is mainly because of the formation of solid electrolyte interphase (SEI) layer.26,29 There are two weak cathodic peaks at 0.95 and 0.86 V observed in the second cycle, which are ascribed to the reductions of Fe3+ to Fe2+ and of Fe2+ to Fe0, respectively, and the lithium insertion into Fe2O3 crystal structure. In the third cycle, the main cathodic peak at ∼0.86 V indicates the reversible insertion of lithium ions and the reduction from Fe2+ to Fe0.26,30 However, the distinct anodic peaks at around 0.1 V are ascribed to the

different processes (Figure 5b). There is no obvious D band observed for both graphite and FeCl3-GICs due to their perfect structures. However, the intercalation of FeCl3 leads to the shift of G band from 1580 to 1621 cm−1.14,36 The G band returns to ∼1580 cm−1 for α-Fe2O3/FLG hybrids due to the disappearance of the charge transfer. The broad and weak band at ∼1315 cm−1 is mainly ascribed to the typical magnon scattering band of α-Fe2O3 (1312 cm−1).25,30,40 The defects induced by the anchored α-Fe2O3 also contribute to the D band. The D band becomes even weaker and partly shifts to 1350 cm−1 for αFe2O3/FLG-10 hybrid, indicating the low content of α-Fe2O3 and fewer defects. The elemental composition and oxidation state of α-Fe2O3/ FLG-5 hybrid are analyzed with XPS (Figure 5c−f). The presences of Fe (2.1%), O (9.3%), C (87.7%), and Cl (0.8%) in the hybrid are reflected by the peaks at 710.9 eV (Fe 2p), 532.3 eV (O 1s), 284.8 eV (C 1s), and 198.8 eV (Cl 2p) (Figure 5c). According to the XRD and aforementioned Raman results, it is reasonable to speculate that most of FeCl3 in the FeCl3-FLG hybrid are converted to α-Fe2O3 after the thermal annealing process. Fe 2p3/2 and Fe 2p1/2 are indicated by the Fe 2p broad peaks at 710.8 and 724.5 eV, respectively,21,41 suggesting the formation of Fe−OH and Fe−O bonds in α-Fe2O3. The peak at 712.6 eV, corresponding to C−O−Fe bond, is an indicator of 11030

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

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ACS Applied Materials & Interfaces

Figure 6. (a) CV curves of the α-Fe2O3/FLG-5 hybrid; (b) discharge/charge curves of α-Fe2O3/FLG-5 hybrid at 200 mA g−1; (c) cycling performances at 200 mA g−1 and (d) rate capabilities at different rates of α-Fe2O3/FLG hybrids and α-Fe2O3/GICs hybrid; and (e) cycling performance of the α-Fe2O3/FLG-5 hybrid at 1000 mA g−1.

lithium deintercalation from graphene sheets.30,41 During the first anodic scan, there are two broad peaks at 1.60 and 1.85 V, corresponding to the oxidations of Fe0 to Fe2+ and of Fe2+ to Fe3+, respectively.18,26,30 The anodic peaks show negligible changes in the subsequent cycles, implying the satisfactory reversibility of the electrochemical reactions. The discharge and charge curves of the α-Fe2O3/FLG-5 electrode (Figure 6b) show a discharge potential plateau at ∼0.9 V and a charge potential plateau at ∼1.8 V,44,45 which are consistent with the CV curves. Note that the specific discharge/charge capacities are as high as 1257.9/861.7 mA h g−1 with a relatively low irreversible capacity loss of 31.5%. The electrochemical properties of α-Fe2O3/FLG electrodes for LIBs are investigated. Figure 6c,d presents the rate capacities and cycling performances of the α-Fe2O3/FLG and α-Fe2O3/GICs hybrids. The α-Fe2O3/FLG-5 hybrid exhibits high initial discharge/charge capacities of 1257.9/861.7 mA h g−1 at 200 mA g−1 with an initial Columbic efficiency of 68.5%. After 350 cycles, the cell shows discharge/charge capacities of 1111.6/1100.5 mA h g−1, with gradual increase of capacity with the cycling number. The slight increase with the cycling is mainly attributing to the activation process. Similarly, α-Fe2O3/

FLG-10 hybrid has discharge/charge capacities of 1193.2/790.4 mA h g−1 with an initial Columbic efficiency of 66.2%, and after 300 cycles, its discharge/charge capacities are still 921.7/921.3 mA h g−1. However, the discharge/charge capacities of αFe2O3/GICs hybrid electrode are 1543.0/969.5 mA h g−1 in the first cycle and become 812.2/808.8 mA h g−1 after 300 cycles. Above all, α-Fe2O3/FLG-5 shows a higher capacity than others on the whole and is more stable in the long-term cycling. The different hybrids are also compared in terms of rate capability from 0.2 to 2 A g−1 (Figure 6d). The α-Fe2O3/FLG-5 hybrid exhibits charge capacities of 831.8, 766.3, 633.9, and 522.8 mA h g−1 after 10 cycles at 0.2, 0.5, 1, and 2A g−1, respectively. It is noteworthy that a high capacity of 969.6 mA h g−1 is recovered after the current density reverts to 0.2 A g−1, confirming the stable structure and the excellent reversibility. Relatively high reversible capacities of 740.5, 639.9, 512.3, 420.8, and 761.5 mA h g−1 are also observed for α-Fe2O3/FLG10 at the current densities of 0.2, 0.5, 1, 2, and 0.2 A g−1, respectively. The electrochemical performances of α-Fe2O3/ FLG-5 are better than those of α-Fe2O3/FLG-10, which is due to the higher content of α-Fe2O3 in the former (48.3 wt %) than in the latter (42.0 wt %), suggesting the proper content of 11031

DOI: 10.1021/acsami.7b00808 ACS Appl. Mater. Interfaces 2017, 9, 11025−11034

Research Article

ACS Applied Materials & Interfaces Fe2O3 is crucial.22,25 In contrast, α-Fe2O3/GICs hybrid prepared by directly heating FeCl3-GICs exhibits an obviously inferior rate performance compared to α-Fe2O3/FLG hybrids, as evidenced by the lower discharge capacities of 774.3, 573.2, 413.5, 308.1, and 627.8 mA h g−1 at 0.2, 0.5, 1, 2, and 0.2 A g−1, respectively. The long-term cycling properties of the α-Fe2O3/FLG-5 hybrid at 1000 mA g−1 are also explored (Figure 6e). The first discharge/charge capacities are 1020.4/684.2 mA h g−1, and they become 658.5 and 650.1 mA h g−1 after 200 cycles with capacity retentions of 95%, suggesting a satisfactory cycling performance for α-Fe2O3/FLG hybrid anode at the high current density. The excellent electrochemical performances of the α-Fe2O3/FLG anode are highly comparable to or even better than those of Fe2O3/carbon hybrid anodes reported in the literature, such as the RGO/Fe2O3 composite prepared by a microwave approach with 1027 mA h g−1 at 100 mA g−1 after 50 cycles (based on the mass of Fe2O3 in the composite),27 the core−shell nanohollow-Fe2O3@graphene fabricated using a Kirkendall process with ∼1050 mA h g−1 at 100 mA g−1,46 the graphene-Fe3O4@carbon fabricated with a self-assembly approach with 860 mA h g−1 after 100 cycles at 100 mA g−1,47 a Fe2O3-carbon nanotube-graphene foam prepared by a CVD and hydrothermal approach with ∼1000 mA h g−1 after 300 cycles at 200 mA g−1,30 and the Fe2O3-carbon nanotubegraphene hybrid by a CVD method with 812 mA h g−1 after 100 cycles at 74.4 mA g−1.48 The remarkable capability and satisfactory cycling stability of α-Fe2O3/FLG hybrid are ascribed to its unique sandwich-like structure consisting of highly conducting FLG sheets and sandwiched α-Fe2O3 particles.49,50 First, the high-quality graphene with high electronic conductivity is beneficial for improving the conductivity of the α-Fe2O3/FLG hybrid. Second, the sandwich-like structure not only buffers the volume change of α-Fe2O3 during delithiation processes51 but also facilitates the fast penetration of electrolyte,44,45 leading to a rapid diffusion of lithium ions and electrons.27,47,52 Third, the intercalated α-Fe2O3 particles have dual functions, preventing the restacking of graphene sheets and enlarging the intragallery of graphite, hence providing active sites for the accommodation of Li-ions.47,51,53 Fourth, the small size of the Fe2O3 particles shortens the diffusion distances of lithium ions and electrons.50 Last but not least, the covalent binding of graphene with Fe2O3 by C−O−Fe linkage offers a good electron transport pathway and a strong interaction during discharge/charge cycles.41 The high-speed Li-ion and electron pathways are responsible for the high rate performance. In contrast, α-Fe2O3/GICs hybrid shows a relatively low reversible capability and a poor rate capability because of the blocked electronic and ionic transport. Thus, the efficiency and scalability of the HPH approach along with the excellent electrochemical properties make the αFe2O3/FLG hybrid promising for application for high-performance LIBs. Furthermore, the HPH approach can be extended to fabricate various metallic compounds supported on graphene with extensive application for energy storage,54 electromagnetic shielding,55 and so on.

exfoliation to high-quality graphene sheets during the HPH process with the low intensity ratio of ID/IG (