Gas-liquid interfacial assembly and electrochemical properties of 3D highly dispersed α-Fe2O3@graphene aerogel composites with a hierarchical structure for applications in anodes of lithium ion batteries

Accepted Manuscript Title: Gas-liquid interfacial assembly and electrochemical properties of 3D highly dispersed ␣-Fe2 O3 @graphene aerogel composites with a hierarchical structure for applications in anodes of lithium ion batteries Author: Jing-Ke Meng Lin Fu Yu-Shan Liu Guang-Ping Zheng Xiu-Cheng Zheng Xin-Xin Guan Jian-Min Zhang PII: DOI: Reference:

S0013-4686(16)32586-5 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.039 EA 28511

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

24-7-2016 20-11-2016 7-12-2016

Please cite this article as: Jing-Ke Meng, Lin Fu, Yu-Shan Liu, Guang-Ping Zheng, Xiu-Cheng Zheng, Xin-Xin Guan, Jian-Min Zhang, Gas-liquid interfacial assembly and electrochemical properties of 3D highly dispersed ␣-Fe2O3@graphene aerogel composites with a hierarchical structure for applications in anodes of lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Gas-liquid interfacial assembly and electrochemical properties of

3D highly dispersed α-Fe2O3@graphene aerogel composites with a hierarchical structure for applications in anodes of lithium ion batteries

Jing-Ke Menga, Lin Fua, Yu-Shan Liua, Guang-Ping Zhengb, Xiu-Cheng Zhenga,c*, Xin-Xin Guana, Jian-Min Zhanga a

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China

b

Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China c

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

*Corresponding author: Tel: +86-371-67783126. E-mail address: [email protected] (X.C. Zheng)

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Graphical Abstract

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Highlights 

3D α-Fe2O3@GA composites are assembled via a gas-liquid interfacial reaction method.

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The composites possess a unique hierarchical structure and high contents of dispersed Fe2O3.

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The composite electrodes exhibit high specific capacity and stable cycling performance.

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The composite electrodes have excellent rate-capability.

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ABSTRACT: Three-dimensional (3D) a-Fe2O3@graphene aerogel (a-Fe2O3@GA) composites with an a-Fe2O3 content as high as 54 wt% were prepared via a gas-liquid interfacial assembly method. It was found that the a-Fe2O3 nanoparticles were well dispersed and embedded into the 3D porous structure of graphene aerogel (GA). Compared with that (SBET=82 m2 g–1) of pristine a-Fe2O3, the resulting composites exhibited much higher specific surface area (SBET=261 m2 g–1). The composites used as anode materials for lithium ion batteries maintained a high reversible capacity of 745 mAh g–1 at a current density of 100 mA g–1 or more than 240 mAh g–1 at a current density of 2000 mA g–1 after 100 cycles, and exhibited superior stable electrochemical cycling performance and excellent rate capability. The superior electrochemical performances were attributed to the synergistic effects of the unique 3D porous structure of the composites with high contents of well dispersed a-Fe2O3. Keywords: a-Fe2O3@graphene aerogel; gas-liquid interfacial reaction; hydrothermal assembly; electrochemical performance; lithium ion battery.

1. INTRODUCTION Because of the increasing energy consumption and the urgent needs for clean energy sources over the world, environmental-friendly energy storage systems have attracted tremendous attention. Among various energy storage devices, rechargeable lithium-ion batteries (LIBs) have the advantages of high energy density, long cycle life and memory-free effect, leading to their widespread applications in portable electronics and electrical tools and machines, especially in electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) [1-4]. Although graphite is 4

the most important commercial anode material for LIBs, it possesses a low theoretical capacity of 372 mAh g–1 with an observed value of around 280-330 mAh g–1, which has limited its applications in high-power LIBs [3,5]. As a consequence, advanced anode materials for LIBs with higher energy density and power density have to be developed. Transition-metal oxides such as NiO [6-10], CuO [11-15], cobalt oxides (Co3O4 and CoO) [16-21], manganese oxides (MnO2, Mn2O3, Mn3O4 and MnO) [22-31], and iron oxide (Fe2O3 and Fe3O4) [32-41] used as anode materials for LIBs are of much interest and have been extensively studied because of their excellent gravimetric capacities. Among the transition-metal oxides, Fe2O3 has attracted considerable research focus owning to its high theoretical capacities (1007 mAh g–1), good stability, environmental friendliness and natural abundance [42]. However, the severe volume change and low electrical conductivity of Fe2O3 electrodes as well as their irreversible phase transformation during lithium ion insertion/extraction could result in severe capacity decay and rate-capability depression. In addition, fractures in the Fe2O3 electrodes provide new sites for corrosion cracking, resulting in consumptions of much more organic electrolytes and lithium ions and the formation of thick solid electrolyte interphase (SEI) films which further hinder the diffusion of lithium ions [43]. To solve the aforementioned problems of LIB anodes made from Fe2O3, composites consisting of Fe2O3 nanoparticles and carbon nanomaterials especially graphene have been widely used to fabricate anode materials for LIBs. Those composites not only significantly improve the electrical conductivity of anodes but also effectively buffer the large volume change of Fe2O3 nanoparticles during charging and discharging processes [5,44-49]. Compared with two-dimensional (2D) graphene, three-dimensional (3D) graphene aerogels 5

(GAs) are more capable of facilitating ion and mass transport and are more desirable for applications in electrode materials [50]. Although the applications of composites consisting of Fe2O3 and GA (Fe2O3@GA) in anode materials for LIBs have been reported in literature [51-53], to the best of our knowledge, those composites are prepared with a one-pot hydrothermal procedure which results in a low content of Fe2O3 incorporated into the poorly structured GA. As a consequence, they are not suitable for applications in anode materials for LIBs since the anodes with a high density of effective electrode materials (such as Fe2O3) are critical for industrial applications of LIBs. In the present work, we synthesize a-Fe2O3@GA composites with a high content (59 wt%) of Fe2O3 via a gas-liquid interfacial reaction under hydrothermal conditions and a subsequent freeze-drying treatment. The as-synthesized a-Fe2O3@GA composites possess a meso- and macroporous structure and a surface area as high as 261 m2 g–1. Remarkably, compared with a-Fe2O3, the a-Fe2O3@GA composites exhibit improved electrochemical cycling stability and excellent rate capability when they are used as anode materials for LIBs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of a-Fe2O3@GA composites Details of the preparation of graphene oxides (GOs) have been reported in our previous study [50]. Scheme 1 shows the schematic illustrations of the system and process for the synthesis of Fe2O3@GA composites. In brief, 5.0 mL of GO solution (~4.0 mg mL–1) was diluted to 2.0 mg mL–1 with ethylene glycol and then ultrasonicated for 1 h, followed by rotary evaporation to remove water. In a 25 mL beaker, 1.20 g Fe(NO3)3·9H2O was dissolved in 5.0 mL of ethylene 6

glycol, and then the diluted GO solution was added dropwisely under vigorous stirring and the stirring was kept for 6 h. Subsequently, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave that contained 6.0 mL of ammonia solution and retained at 180 ◦C for 24 h. The resulting a-Fe2O3@graphene hydrogel (a-Fe2O3@GH) composites were thoroughly washed with deionized water and ethanol, followed by a freeze-drying process to obtain the a-Fe2O3@GA composites. The mass percentage of a-Fe2O3 in the composites was as high as 59%. For comparison, GA and a-Fe2O3 were synthesized by the same method without Fe(NO3)3·9H2O and GO, respectively. 2.2 Structure characterization Thermogravimetric (TG) analysis was measured by the TG instrument (BJ HENVEN) with a heating rate of 10 ◦C min–1 in air to determine the chemical composition of the resulting composites. The porous nature of the samples was investigated by nitrogen adsorption-desorption isotherms at -196 ◦C using a Micromeritics ASAP 2420 surface area and porosity analyser. The specific surface area was calculated from the nitrogen adsorption isotherms within the relative pressure range of 0.05-0.25 by the Brunauer-Emmett-Teller (BET) method. The pore-size distributions were measured from the adsorption branches by using the Barrett−Joyner−Halenda (BJH) method. The microscopic features of the Fe2O3@GA composites were examined using a Zeiss Ultra 55 scanning electron microscope (SEM) and a JEM-2100 transmission electron microscopy (TEM). Crystal structures of the synthesized samples were characterized by X-ray diffraction (XRD) on a Panalytical X’pertPro diffractometer operated at 40 kV and 40 mA using Cu Kα radiation (λ=0.154 nm). Surface chemistry was studied by X-ray photoelectron spectroscopy (XPS) using an RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Al Ka 7

radiation (hν=1486.6 eV). Binding energies were calibrated using the containment carbon (C 1s=284.6 eV). Raman spectra recorded on a Renishaw RM-1000 spectrometer and Fourier-transform infrared (FT-IR) spectra recorded on a Thermo scientific Nicolet 380 Fourier transform spectrometer using a KBr pellet technique were used to characterize the samples. 2.3 Electrochemical measurements Electrochemical performances of the electrode materials for lithium ion batteries were tested in lithium cells (CR 2016 coin type cell). The working electrodes were prepared by mixing 80 wt% of active materials, 10 wt% of super P and 10 wt% of polyvinylidene fluoride (PVDF) with N-methylpyrrolidinone (NMP) solvent to form a slurry, which was coated onto copper foil and dried at 120 ◦C for 12 h in a vacuum oven. The loading mass of Fe2O3@GA composites is 0.8 mg cm-2. The cells were assembled in an argon-filled glove box using active materials as the working electrode, a lithium foil as the counter electrode and a celgard 2400 microporous membrane as the separator. The organic electrolyte was prepared by dissolving 1.0 moL L–1 LiPF6 in a mixture solution of ethylene carbonate, diethyl carbonate and ethyl methyl (volume ratio of 1 : 1 : 1). Galvanostatically charge-discharge characteristics were tested on a LAND battery system between 0.01 and 3.00 V at different current densities. Cyclic voltammetry tests were performed on a CHI660C electrochemistry workstation at a scan rate of 0.2 mV s–1 over a potential range of 0.01 to 3.00 V (vs. Li/Li+) at room temperature.

3. RESULTS AND DISCUSSION The XRD patterns of GA, the a-Fe2O3@GA composites and a-Fe2O3 are shown in Figure 1a. The characteristic peaks of Fe2O3 sample are in good agreement with that of hematite Fe2O3 8

(a-Fe2O3, JCPDS 33-0664) [51,54]. The strong diffraction peaks indicate that the a-Fe2O3 sample is highly crystallized. The diffraction patterns of the a-Fe2O3@GA composites are also well consistent with those of the hematite phase of Fe2O3. No obvious shifts of the peaks can be observed for the a-Fe2O3@GA composites as compared with those of the hematite phase of Fe2O3, meaning that the strategy employed in the present work for the synthesis of composites does not change the crystal structure of α-Fe2O3. The weakened intensities of the XRD peaks for the a-Fe2O3@GA composites are caused by the additions of GA. Figure 1b shows the TG curves of samples. The bare Fe2O3 exhibits a weight loss of about 5% below 250 ℃, which may be ascribed to the loss of its adsorbed species such as NH3 and water. On the contrary, almost 100% weight loss can be found in the case of GA. The Fe2O3@GA composites exhibit a total weight loss of about 41% below 500 ℃, which is attributed to the evaporation of adsorbed water molecules and the combustion of GA. Therefore, the content of Fe2O3 in the composites is ~54 wt%, which is much higher than that in Fe2O3@GA derived from Fe(OH)3 colloids [55]. It can be observed in the TG curve that the oxidation temperature of Fe2O3@GA decreases to about 380 ℃, while pure GA starts to burn drastically at 530 ℃. The large decreases in those temperatures indicate that there is strong interaction between Fe2O3 and GA, which is similar to that between a-Fe2O3 and RGO [56]. The surface areas and the porous structures of GA, a-Fe2O3 and a-Fe2O3@GA are determined by N2 adsorption-desorption analysis. As shown in Figure 2 a, for pristine a-Fe2O3, the hysteresis loop occurs at a relative high pressure (P/P0) ranging from 0.8 to 1.0 should be classified as the type V according to the IUPAC classification, indicating that the pores in a-Fe2O3 could mainly result from the void spaces among the nanoparticles. GA gains an increase in adsorption due to 9

capillary condensation, which is associated with a distinct hysteresis loop at P/P0=0.35 -0.75. The results indicate that there are mesopore channels inside GA. The curve for the a-Fe2O3@GA composites is similar to that of GA, demonstrating that the incorporation of a-Fe2O3 nanoparticles has not obvious effects on the pore structure of GA. In addition, there are sharp increases at low relative pressures (P/P0 ＜ 0.10) in the isotherms for both GA and a-Fe2O3@GA, indicating they have abundant microspores. As shown in Figure 2b, a-Fe2O3@GA retains the characteristics of pore-size distribution of GA. On the contrary, a-Fe2O3 exhibits a wide pore-diameter distribution. The results demonstrate that the a-Fe2O3 nanoparticles are highly dispersed inside GA. The detailed textural and structural characteristics of GA, a-Fe2O3 and a-Fe2O3@GA samples are listed in Table 1. Compared to a-Fe2O3 with a low specific surface area of SBET=82 m2 g–1, the a-Fe2O3@GA composites exhibit much higher surface area (SBET=261 m2 g–1). This is mainly attributed to the existence of the interconnected 3D aerogel structure and the presence of GA (SBET=362 m2 g–1) in the composites. The large specific surface area and relatively narrow pore-diameter distribution not only offer enough open channels as passages of ions and electrons for the electrochemical reactions but also shorten the ion diffusion paths and improve the utilization of active materials. The microstructure and morphology of the resulting composites are further confirmed by SEM. Compared with those of severally aggregated a-Fe2O3 nanoparticles (Figure 3a, b), the SEM images of the resulting a-Fe2O3@GA composites show an interconnected 3D network structure, in which the dense a-Fe2O3 nanoparticles are anchored uniformly into GA (Figure 3c, d) and the particle size (30-70 nm) is consistent with that of the average crystallite size (46 nm) calculated from the broadening of XRD peaks according to Scherrer’s equation. The dense loading of 10

a-Fe2O3 nanoparticles on graphene sheets is further shown in the TEM images. As shown in Figure 4a and b, it can be seen that the a-Fe2O3 nanoparticles are tightly anchored to the surfaces and the adjacent layers of graphene sheets to form a porous and conductive network architecture, even the composites are treated by strong sonication in ethanol before TEM measurement. The high-resolution TEM (HRTEM) image of a-Fe2O3@GA is shown in Figure 4c, which also confirms the integration of the well-crystallized a-Fe2O3 nanoparticles with graphene sheets. Furthermore, the corresponding selected area electron diffraction (SAED) patterns shown in the inset of Figure 4c display well-resolved diffraction rings, which can be indexed as those of a-Fe2O3 and graphene. In addition, the photograph of the a-Fe2O3@GA composites shown in the inset of Figure 3b indicates that they have a 3D macroscopic shape. The above-mentioned SEM analysis demonstrates that there exist abundant well-separated a-Fe2O3 nanoparticles and pores in the composites, in consistent with those results determined from the N2 adsorption-desorption measurements. Such microstructure could result from the efficient assembly of a-Fe2O3 nanoparticles in GA during the gas-liquid interfacial reaction under the hydrothermal conditions, and could play an important role in enhancing the electrochemical performance of a-Fe2O3@GA. Reasons are explained as follows: First, as demonstrated by the TEM analysis, the tight contact between a-Fe2O3 nanoparticles and graphene sheets enables the fast electronic and ionic transport through the active materials to the collector and thus improves the electrochemical performance. Second, a-Fe2O3 nanoparticles can act as spacers and effectively separate the graphene sheets from each other, avoiding the loss of the active surface area of GA. Third, the graphene layers with excellent mechanical strength and flexibility could not only effectively prevent the a-Fe2O3 nanoparticles from aggregation but also accommodate the volume 11

expansions of the a-Fe2O3 nanoparticles, guaranteeing a continuous electrical conductive network in the electrode material during electrochemical cycling. XPS analysis is performed to determine the surface composition of the a-Fe2O3@GA composites. The survey spectrum of the a-Fe2O3@GA composites is shown in Figure 5a. The peaks at about 284.6 and 532.0 eV are assigned to the elements of C and O, respectively. In addition, the peak at 395.8 eV is assigned to the element of nitrogen, which comes from the ammonia solution in the gas-liquid interfacial reaction during the synthesis of a-Fe2O3@GA. The existing nitrogen may reduce the energy barrier of lithium penetration and increase reactive sites, improving the lithium storage properties to some extent [57,58]. The spectrum of Fe 2p shown in Figure 5b displays two broad peaks with satellite peaks at 726.8 and 713.2 eV, indicating the existence of Fe3+ [59]. The FT-IR spectrum of a-Fe2O3@GA is shown in Figure 5c. The FT-IR peaks for GA and a-Fe2O3 can be found to coexist in the a-Fe2O3@GA composites. As for GA, those peaks corresponding to the oxygen functionalities, such as the C=O stretching vibration peak at 1725 cm−1 and the C–O (alkoxy) stretching peak at 1045 cm−1, disappear entirely. While the skeletal vibration band for the sp2 domains at about 1622 cm–1 remains. These observations confirmed that GO is well reduced into graphene [50]. For a-Fe2O3, the bands appeared at 574 and 476 cm–1 can be assigned to the stretching vibrations of the Fe3+-O2– bonds in the FeO6 octahedron and FeO4 tetrahedron, respectively [60]. The XPS and FT-IR results thus confirm the successful incorporation of a-Fe2O3 into GA [55]. Raman spectra of GA and the a-Fe2O3@GA composites are presented in Figure 5d. Both of them exhibit two well-known peaks at round 1330 and 1592 cm–1, corresponding to D-band (defect band) which is considered as a breathing mode of k-point phonons of A1g symmetry and 12

G-band (graphite band) which is derived from the E2g phonon of C sp2 atoms, respectively [61]. It is known that the intensity ratio of the D- and G- bands (ID/IG) reflects the graphitization degree of carbon materials. The calculated ID/IG=1.33 for the a-Fe2O3@GA composites is higher than that of GA (1.13), implying that the incorporation of a-Fe2O3 nanoparticles increases the defects in graphene. Figure 6a show the initial three representative charge-discharge curves for the a-Fe2O3@GA composites used as anode materials for LIBs at a current density of 100 mA g–1. Obvious differences between the first and the subsequent two cycles can be observed. There is a distinct voltage plateau at around 0.83 V in the discharge curve while it is at 1.69 V in the first cycle charge curve, which is attributed to the reversible redox of Fe2O3 with lithium ions (Fe2O3+6Li++6e–1↔2Fe+3Li2O) [62]. In addition, the voltage below 0.58 V in the first discharge curve is related to the formation of LiC6, accompanied by some irreversible reactions [63]. The initial discharge specific capacities of a-Fe2O3@GA, Fe2O3 and GA are found to be 1749, 1408 and 1220 mAh g–1, and their respective charge capacities are 809, 190 and 302 mAh g–1, respectively, with an initial coulombic efficiency of around 46.3%, 13.5% and 24.8%. In contrast to those of pure Fe2O3 (Figure S1a in the Supporting Information), the next two cycles of the discharge/charge profiles of a-Fe2O3@GA are almost identical with a much higher coulombic efficiency (above 90%), indicating the high reversibility of the anode materials. The higher initial coulombic efficiency is mainly due to the synergistic interactions between Fe2O3 and GA, resulting in efficiently increased surface electrochemical activity and decreased polarization. Figure 6b shows the cyclic voltammograms profiles for the a-Fe2O3@GA composites in the initial three cycles. In the first cycle, three cathodic peaks at 1.52, 1.33 and 0.69 V can be 13

observed, corresponding to the formation of Li-intercalated hexagonal phase (a-LixFe2O3), Li-intercalated cubic phase (Li2Fe2O3) and the reduction of Fe2+ to Fe0, respectively, accompanied by some irreversible reactions such as the decomposition of electrolyte and the formation of SEI films [64-66]. In addition, the broad peak near 0.1 V may be related to the Li+ ion insertion in the matrix of GA and the lithium storage at the active sites such as edge-type sites and nanopores in GA [63]. While the broad anode peak at around 1.74 V represents the oxidation reaction of Fe0 to Fe3+. In the two cycles that follow, the initial peaks at 1.52 V and 1.33 V disappear as a result of the irreversible phase transformation in the first cycle. Compared to those of Fe2O3, the shift and the decreased intensities of the redox peak of a-Fe2O3@GA after the first cycle is due to the high electrochemical reversibility, indicating that the electrode materials are gradually stabilized and the polarization is reduced (Figure S1 c,d in the Supporting Information,) [42, 43]. To further investigate the electrochemical performance of the a-Fe2O3@GA composites, the rate capabilities are also evaluated at different current densities (Figure 6c). The reversible capacity of the resulting anode is about 745 mAh g–1, 609 mAh g–1, 461 mAh g–1, 302 mAh g–1 and 240 mAh g–1 at current densities of 100 mA g–1, 200 mA g–1, 500 mA g–1, 1000 mA g–1 and 2000 mA g–1, respectively. Although the specific capacity of the composites has significant capacity fading at an increased current density, it is higher than those of Fe2O3 and GA (Figure S2 in the Supporting Information,). The discharge capacity and the current density of the composites can be as high as 240 mAh g–1 (~ 192 μAh cm–2) and 2000 mA g–1, respectively. Once the current density is returned to the initial value of 100 mA g–1 after 50 cycles, the Fe2O3@GA composites still possess a high specific capacity (856 mAh g–1), which could be attributed to their stable microstructure and excellent electrical conductivity during the discharge-charge process. 14

Figure 7a shows the electrochemical cycling performance of a-Fe2O3, GA and the a-Fe2O3@GA composites over 100 cycles at a current density of 100 mA g–1. All anode materials show good cycling steadily. The discharge capacity of pristine a-Fe2O3 fades obviously after 5 cycles, with the discharge capacity ranging from 1409 mAh g–1 to 68 mAh g–1. The low specific capacity and rapid capacity fading of the a-Fe2O3 electrode during cycles indicate that it cannot meet the practical applications. Compared to a-Fe2O3 and GA, the a-Fe2O3@GA composites exhibit better cycling stability and higher specific capacity. The discharge capacity of the a-Fe2O3@GA composites is retained at about 745 mAh g–1 (~ 596 μAh cm–2) at 100 mA g–1 after 100 cycles, which is much higher than those of a-Fe2O3 (69 mAh g–1) and GA (207 mAh g–1). It is also higher than the theoretical specific capacity (Ctheo.=CFe2O3,theo.×Fe2O3%+CGA,theo.×GA%=1007×54%+372×46%=715 mAh g–1) of a simple mixture of a-Fe2O3 and GA, mainly resulting from the synergistic interactions between a-Fe2O3 and GA described above. It should be pointed out that the loading density of a-Fe2O3@GA is estimated to be about 0.8 mg cm–2, which seems to be a common problem associated with light-weight graphene materials [5,54]. To clarify the differences in electrochemical performances among Fe2O3, GA and Fe2O3@GA, EIS measurements are conducted to have a better understanding on the improved electrochemical properties of the composites. As shown in Figure 7b, the Nyquist plots of the three samples consist of a semi-circle in the high-frequency region assigned to charge-transfer resistance, and an inclined line in the low-frequency region representing the Warburg impedance. Apparently, the diameter of semicircle for the anode made from the Fe2O3@GA composites is

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significantly smaller than that of Fe2O3, meaning that the Fe2O3@GA anode exhibits a higher electrical conductivity and a lower charge-transfer resistance than those of Fe2O3. The excellent cycling performance and capacity retention of the a-Fe2O3@GA composites can be attributed to the synergistic interaction between a-Fe2O3 and GA, which may be due to the following possibilities: (i) The hierarchical 3D architecture of graphene aerogels not only provides a support for anchoring highly dispersed a-Fe2O3 nanoparticles, but also accommodates the volume change during the electrochemical cycling process, prevents the pulverizing of the Fe2O3 electrode during lithiation/delithiation, and enhances the electrical conductivity of the overall electrode during the electrochemical process. In addition, the existence of Fe2O3 nanoparticles anchored on graphene sheets can prevent the restacking of graphene sheets, which is favorable for increasing the active surface area and the lithium storage capacity of graphene in the composites; (ii) The unique microstructure of the Fe2O3@GA composites can ameliorate the lithium ion accessibility in its mesoporous structure and shorten the lithium ion diffusion length during the insertion/extraction processes and enhance the utilization of active materials.

4. CONCLUSION We have successfully synthesized 3D a-Fe2O3@graphene aerogel (a-Fe2O3@GA) composites via a gas-liquid interfacial assembly method under hydrothermal conditions. The content of a-Fe2O3 nanoparticles uniformly anchored on the graphene sheets in the resulting composites is as high as 54 wt%. The a-Fe2O3@GA composites possess a much larger surface area (SBET=261 m2 g–1) compared with that of a-Fe2O3 (SBET=82 m2 g–1). The composites used as anode materials for LIBs show excellent electrochemical cycling and rate performances which could be 16

related to their inherent 3D aerogel structure, delivering a high reversible capacity of 745 mAh g–1 (or 596 μAh cm–2) after 100 cycles. This work thus demonstrates that the a-Fe2O3@GA composites are promising candidates of anode materials for LIB applications.

ACKNOWLEDGEMENT The authors are grateful for the supports of the National Natural Science Foundation of China (No. U1304203), the Natural Science Foundation of He’nan Province, the Foundation of He’nan Educational Committee (No. 16A150046) and the Innovation Foundation of Zhengzhou University (Nos. 201610459062 & 2016xjxm251). GPZ is grateful for the support provided by the Research Grants Council of Hong Kong SAR, China (No. 15260716).

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Table 1. Structural characteristics of GA, Fe2O3 and the Fe2O3@GA composites

SBETa

SMb

SExc

Vpd

VMe

Daf

Dag

Dah

(m2 g–1)

(m2 g–1)

(m2 g–1)

(cm3 g–1)

(cm3 g–1)

(nm)

(nm)

(nm)

GA

362

79

283

0.228

0.0370

2.52

2.84

2.77

Fe2O3

82

—

83

0.356

0.016

17.40

16.87

15.13

Fe2O3@GA

261

32

229

0.191

0

2.93

3.22

3.09

Sample

a

SBET, BET surface area.

b

c

SEx, t-Plot external surface area.

d

e

f

SM, t-Plot micropore area.

Vp, Single point adsorption total pore volume.

VM, t-Plot micropore volume.

Da, Adsorption average pore width (4V/A by BET).

g

Da, BJH Adsorption average pore diameter (4V/A).

h

Da, BJH Desorption average pore diameter (4V/A). 24

(a)

(b) Scheme 1. Schematic illustrations of the system (a) and process (b) for the synthesis of the Fe2O3@GA composites.

25

Figure 1. XRD patterns (a) and TGA curves (b) for the materials.

26

Figure 2. N2 adsorption-desorption isotherms (a) and pore-size distributions (b) of GA, Fe2O3 and the Fe2O3@GA composites.

27

Figure 3. SEM images of Fe2O3 (a, b) and the Fe2O3@GA composites (c, d).

28

Figure 4. TEM images (a-c) and EDX spectrum (d) of the Fe2O3@GA composites.

29

Figure 5. XPS survey spectrum (a) and high-resolution Fe 2p spectrum (b) of the Fe2O3@GA composites, FT-IR spectra (c) and Raman spectra (d) of the materials.

30

Figure 6. Discharge-charge profiles (a), CV curves (b), and rate capability (c) of the Fe2O3@GA composites.

31

Figure 7. Cycling performance (a) and Nyquist plots (b) of Fe2O3, GA and the Fe2O3@GA composites.

32