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Efficient reduced graphene oxide grafted porous Fe3O4 composite as a high

Subramani Bhuvaneswari,a Parakandy Muzhikara Pratheeksha,a Srinivasan Anandan,a* Dinesh Rangappa,a Raghavan Gopalan,b Tata Narasinga Rao a a

Centre for Nano Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad-500 005, India b

Centre for Automotive Energy Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Chennai-600 113, India

Abstract Here, we report a facile fabrication of Fe3O4/reduced graphene oxide (Fe3O4/RGO) composite by novel approach, i.e., microwave assisted combustion synthesis of porous Fe3O4 particles followed by the decoration of Fe3O4 by RGO. The characterization studies of Fe3O4/RGO composite demonstrate the formation of face centered cubic hexagonal crystalline Fe3O4, and the homogeneous grafting of Fe3O4 particles by RGO. The nitrogen adsorption-desorption isotherm shows the presence of porous structure with surface area and pore volume of 81.67 m2 g-1, and 0.106 cm3 g-1 respectively. Raman spectroscopic studies of Fe3O4/RGO composite confirm the existence of graphitic carbon. The electro-chemical studies reveal that composite exhibit high reversible Li-ion storage capacity with enhanced cycle life and high columbic efficiency. Fe3O4/RGO composite showed a reversible capacity ~ 612, 543, and ~ 446 mAh g-1 at the current rates of 1C, 3C and 5C respectively with a columbic efficiency of 98% after 50 cycles, which is higher than graphite, and Fe3O4/carbon composite. The cyclic voltammetry experiment reveals the irreversible and reversible Li-ion storage in Fe3O4/RGO composite during the beginning and the subsequent cycles. The results emphasize the importance of our strategy which 1

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performance anode material for Li-ion batteries

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exhibited promising electro-chemical performance in terms of high capacity retention and good

particles with high surface area and pore volume, and (ii) increased electronic conductivity by RGO grafting attributed to the excellent electro-chemical performance of Fe3O4, which make this material attractive to use as anode materials for lithium ion storage. Keywords: Fe3O4, reduced graphene oxide, porous, electronic conductivity

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cycling stability. The synergistic properties, (i) improved ionic diffusion by porous Fe3O4

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Rechargeable lithium ion batteries (LIBs) are a most attractive power source not only for portable electronic devices1-3 but also intensively pursued for powering upcoming hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). In LIBs, graphite has been used as anode material because lithium can be inserted/extracted during charging and discharging process, resulting in a theoretical specific capacity of 372 mAh/g.4-6 However, the relatively low reversible capacity and poor stability at a higher rate restricts its use in HEV and PHEV applications, where high energy and power density is a prerequisite. Alternative to graphite, transition metal oxides such as NiO,7,8 Fe3O4,9-12 Fe2O3,13,14 SnO2,15 Co3O4,16 and CuO17 have been investigated owing to their high theoretical capacity (~700-1000 mAh g-1) and their capability of intake excess of Li+-ion18 during charge-discharge process. Among these metal oxides, magnetite (Fe3O4) considered as a promising anode material due to its excellent characteristics19a,b including natural abundance, high electronic conductivity, low cost, and environmental benign in comparison with other metal oxides. In addition, it is capable of reacting with eight Li+ ions per formula unit with theoretical capacity of about 926 mAh g-1.20 However, its application hindered by poor cycling performance and volume change occurrence during lithiation/de-lithiation process, resulting in a severe loss of capacity and electrical conductivity.21 In order to address this challenge, well-designed nanostructures iron oxides22a,b have been fabricated, which made the Li+ diffusion in electrodes become easy, leading to significantly improved electro-chemical performance. Particularly, porous electrode materials23 have shown promising candidate for the improvement of LIBs performances, because it not only enhanced Li+-ion diffusion, but also ensured a high electrode-electrolyte contact area and it 3

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1. Introduction

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confers the ability to accommodate volume expansion. Various methods have been reported for

solvothermal,24c hard templating,24d co-precipitation,24e sol-gel,24f oxidation/reduction,24g thermal decomposition,24h,i and colloidal chemistry.24j However, these methods suffer by several disadvantages including slow reaction kinetics, non-uniform reaction conditions, sharp thermal gradients, and scale-up issues.25a,b Moreover, complicated experimental conditions (high reaction temperature and pressure) and tedious operation procedures limit their routine applications. Hence, it is a great challenge to explore a facile, quick and eco-friendly method to prepare porous nano-structured materials. Microwave-chemistry26a proved an efficient method for the preparation of nano-structured materials, and it can be used for the production of homogeneous, high purity, and crystalline oxide powders at significantly lower temperature than the conventional methods.25b, 26b Rapid volumetric heating, high reaction rate, fast reaction time, less amount of external energy, and large scale production of high quality of particles are the advantages of this process.26c Hong et al27a and Wang et al27b synthesized crystalline, spinel magnetite nanoparticles by microwave-assisted process and showed a promising application in magnetic field. Yu et al27c reported a microwave assisted hydrothermal synthesis of Fe3O4/C composites. Recently, surfactant27d and surfactant free27e microwave based process has been developed for the fabrication of Fe3O4 and Cu nanostructures respectively. Cheng et al28 reported the synthesis of a rugated porous Fe3O4 material by multi-pulse electro-chemical anodization. However, unique synthetic approach is a prerequisite to fabricate porous nano-structured Fe3O4 anode materials if one wants to reduce the process cost and improve the performance of LIBs further.

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the synthesis of porous nano-structured Fe3O4, including hydrothermal synthesis,24a,b

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Recently a new type of architecture, i.e., nano-structured electrode materials together with

current rate,29a,b and electronic conductivity.29c Particularly, many groups have investigated hybrid structures; a graphene grafted with anode materials like Fe2O3,30a Fe3O4,30b TiO2,30c SnO2,30d Co3O4,30e and Mn3O4,30f since graphene (two dimensional, sp2 hybridized carbon with honeycomb structure) having high thermal stability, superior electronic conductivity (~3080 W mK-1), greater structural flexibility and large surface area (~2600 m2 g-1).31-36 Though few reports available for the combination of Fe3O4 with graphene based composites, to the best of our knowledge, a combination of porous Fe3O4 particles (prepared by microwave-assisted process) together with reduce graphene oxide composites has not been reported so far. In this article, we will demonstrate a simple, rapid, and environmental benign microwave assisted synthesis of porous Fe3O4 particles followed by the decoration of Fe3O4 by reduced graphene oxide (RGO). The structural characterization of Fe3O4/RGO composites demonstrates the presence of highly crystalline porous Fe3O4 particles with high surface area and pore volume, and uniform grafting of Fe3O4 particles by RGO. The electro-chemical studies indicate that the composite exhibit high reversible Li-ion storage capacity with enhanced cycle life and high columbic efficiency, which is better than the electro-chemical performance reported previously. We found that the synergistic effect; i.e., improved Li+-ion diffusion by porous materials and increased electronic conductivity by RGO dispersion are attributed to the improved electro-chemical performance of Fe3O4 anode materials. The promising Fe3O4/RGO composite developed in the present study have the potential for use as anode materials in the upcoming LIBs and the microwave process used for the synthesis of Fe3O4 can be extended to other electrode materials.

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carbonaceous additives found as an ideal combination to improve the reversible capacity at high

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2.1 Preparation of Reduced Graphene Oxide (RGO) First, Graphene oxide (GO) was prepared from graphite by modified Hummers and Offeman’s method.37 In a typical synthesis, 5 g of graphite, 2.5g of NaNO3, 85ml of HNO3 and 265 ml of H2SO4 was stirred together in an ice water bath. Then 15 g of KMNO4 was added and continued stirring for 2 h. After stirring, the solution was removed from ice bath and kept in a water bath for 0.5 h with a temperature of 30-35˚C. Then, 250 ml of water added slowly into the above solution and stirred for 15 min, followed by 750 ml of hot water and 5 ml of H2O2 was added and stirring continued for 20-30 min. The solution color turns dark brown to yellow after completion of the reaction. Then the solution was filtered and washed with distilled water for several times to remove the water soluble products by centrifugation at high speed rate of 8000 rpm. The filtrate was then dispersed into the distilled water and sonicated for 1 h. To reduce GO to RGO, ascorbic acid was used as a reducing agent. 1 g of ascorbic acid added to the 200 ml of GO solution under magnetic stirring and stirring continued for 2 h at 100˚C. Then the solution was centrifuged and re-dispersed into distilled water, and the resulting solution was sonicated for 0.5 h to get the homogeneous RGO solutions. 2.2 Synthesis of Fe3O4/RGO composite The preparation of Fe3O4/RGO composite involved two steps as shown in Fig.1. In the first step iron oxide particles were prepared by microwave assisted combustion synthesis using a microwave oven (Model: Samsung, CE107FF-S). In a typical synthesis, 2M (16.16 g) of iron nitrate (Fe (NO3)3.9H2O) and 1.66M (2.492 g) of glycine (C2H5NO2) was dissolved in 40 ml of 6

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2. Experimental Section

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mixed solvents (water and ethanol). The water to ethanol ratio is 1:1. The solution was mixed

at 900 W for 10 min. During synthesis, microwave assisted combustion converts the precursor’s solution into porous iron oxide particles. The second step involved the preparation of Fe3O4/RGO composite, in which 23 ml RGO solution was taken to prepare 10 wt. % and stirred for a few min. Then, 0.5 g of iron oxide powder was added into RGO solution which is followed by drying under continuous magnetic stirring. After drying the solution, the powder was collected and carbonized at 650˚C for 10 h under argon atmosphere at the heating rate of 5˚C/min. The 30 wt. % and 50 wt. % of Fe3O4/RGO composite were prepared by only varying the amount of RGO solution using the same procedure. 2.3 Characterization X-ray Diffraction (XRD) measurement was performed with Bruker AXS D8 advance system using Cu Kα radiation (λ=1.5406Å) over the range of 2θ =10-90o at room temperature. The grain size of the material was calculated by Scherrer’s equation: Dp = 0.9λ/β1/2 .cos θ , where Dp is the average grain size in Å, β1/2 the full width of the peak at half maximum, and θ is the diffraction angle. Nitrogen adsorption-desorption isotherms were measured at -196 ˚C on a Micromeritics ASAP 2020. The samples were out-gassed at 250 ˚C for 3 h prior to the nitrogen adsorption measurements. The surface area of the particle was measured using the Brauner- Emmet-Teller (BET) equation and is expressed in m2/g. The pore volume and the pore diameter were measured for the adsorption and desorption separately. Field emission – Scanning electron microscope (FE-SEM) were carried out with Hitachi S-4300 SE/N microscope operating at 20 KV. The materials were coated with a thin gold layer by the sputtering process to make them conducting 7

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and stirred for 10-15 min. The resulting solution was kept inside the microwave oven and heated

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for SEM analysis. The morphology of the materials was performed by high resolution

spectroscopy (XPS) analysis was obtained to analyze the surface chemistry (oxidation states and the elemental compositions) of the materials using the ESCA-Omicron XPS system with a MgKα as the excitation source. The Raman spectra were recorded in the range between 300 and 3000 cm-1 at room temperature by Raman spectrometer (Model: Horiba Jobin Yvon Lab Ram HR-800), using argon ion laser as a source with an excitation wavelength of 514 nm. The quantitative analysis of Fe3O4/RGO composite was measured by Thermo gravimetric (TG) and differential scanning calorimetry (DSC) method. TG and DSC instrument (Model: STA 449 F3 Jupiter NETZSCH, Germany) was used to measure carbon content by weight loss. During analysis, the samples were heated from room temperature to 1000˚C at the heat rate of 10 ˚C/min. under static air. The functional group present in Fe3O4/RGO composite was measured by Fourier transform infrared spectrometer (FT-IR) (Model: vertex 70). 2.4 Electro-chemical measurements The electro-chemical performance of the Fe3O4/RGO composite was evaluated using Swagelok half cell. The working electrode was prepared by mixing Fe3O4/RGO composite, acetylene black (conducting carbon) and polyvinylidenedifluoride (binder) at the weight ratio of 80:10:10 respectively. 1 M solution of LiPF6 in ethylene carbonate/dimethyl carbonate (1:1 v/v) was used as electrolyte. For electrode fabrication, initially Fe3O4/RGO composite, acetylene black and binder were mixed properly using a solvent (N-methyl pyrolidone) and the resulting slurry was coated uniformly on Cu foil followed by drying at 120 ˚C for 12 h. Then the half cell (Swagelok cell) was fabricated inside the argon filled glove box using Fe3O4/RGO composite as working 8

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transmission electron microscope (Model: Tecnai G-20, 200 KV). X-ray photoelectron

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electrode and lithium foil as reference and the counter electrode. The electro-chemical

between 0.01 to 3 V at different current densities. AC impedance spectra of the samples were obtained using impedance/gainphase analyzer (Solartron-1260) in the frequency range of 100 KHz to 0.01 Hz with AC signal amplitude of 5 mV.

3. Results and Discussions 3.1 Characterization Fig. 2A shows the XRD pattern of (a) standard Fe3O4, (b) as-synthesized Fe3O4, (c) heat treated Fe3O4, and (d) Fe3O4/RGO composite. XRD pattern of as-synthesized material (b in Fig. 2A) shows the presence of mixed phase (Fe2O3 and Fe3O4), similar to the XRD pattern of standard Fe2O3 (hematite, JCPDS-04-003-2900) and Fe3O4 (magnetite, JCPDS- 04-007-2718), which is consistent with the XRD pattern of iron oxide reported previously.38 XRD pattern of heat treated Fe3O4 material is shown in Fig. 2A (c). Though the profile matched with the XRD patterns of standard Fe3O4, traces of Fe2O3 are also present. However, XRD pattern of Fe3O4/RGO composite (d in Fig. 2A) after carbonization exhibit pure Fe3O4 cubic hexagonal phase without impurity peaks. It is interesting to note that heat treated iron oxide without RGO shows the pattern of Fe3O4 with traces of Fe2O3, while pure Fe3O4 was obtained in the presence of the RGO, because RGO acts as a reducing agent to reduce Fe2O3 in to Fe3O4 during carbonization.39 These results are in good agreement with the results of XRD pattern reported previously for Fe3O4/C.40, 41

The crystal size of Fe3O4/RGO composite calculated from Scherrer’s equation is 18 nm. The

lattice parameter of Fe3O4/RGO composite (a = 8.3873) similar to the lattice parameters of the face centred (Fd-3m-227) cubic structure of standard magnetite Fe3O4 (JCPDS 04-008-4512, a = 8.3873). Overall XRD results reveal the formation of pure Fe3O4 particles in the presence of 9

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measurement was carried out using Arbin instruments (Model: BT2000) with a potential range

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RGO solution. Fig. 2B shows the Nitrogen adsorption desorption isotherm of (a) Fe3O4 and (b)

pressure (p/p0) and confirms the presence of porous structure.42a,b,c Zhou et al43 investigated the nitrogen adsorption isotherm of Fe3O4-graphene nanosheets and demonstrated that the isotherm obtained between 0.4 to -0.9 relative pressures (p/p0) correspond the presence of mesoporous and macroporous structure with an open porous system. Hence, in the present study it is believed that the material may contain both meso and macro porous structure which is beneficial to decrease the volume expansion that occurs during solid electrolyte interface layer formation. The surface areas and pore volumes of Fe3O4 and Fe3O4/RGO composite are 63.2 and 81.67 m2 g-1, and 0.1225, and 0.106 cm3g-1 respectively. It is worth noting that the RGO addition increased the surface area and pore volume of Fe3O4. The porous structure of Fe3O4/RGO composite with high surface area and pore volume facilitates electrolyte ion diffusion to active sites with less resistance44 and acts as a buffer layer for volume expansion45 of Fe3O4 during charge-discharge process. The porosity and Fe3O4 particles in RGO could shorten the diffusion paths of Li+ and electrons, and more effectively buffer the volume changes during Li+ insertion and extraction reactions. All of these could contribute to improvements in lithium storage performance and cycle stability. FE-SEM images of the Fe3O4, RGO and Fe3O4/RGO composite are displayed in Fig. 3. The distorted spherical like Fe3O4 particles with sizes in the range of 100-300 nm obtained by microwave combustion synthesis (Fig. 3a). The inset of Fig. 3a reveals the formation of porous like Fe3O4 particles, which is due to the liberation of nitric oxide from the precursors during microwave combustion synthesis. Fig. 3b shows the FE-SEM images of the RGO, indicating the formation of sub-micron sized flake RGO after chemical reaction as described in the 10

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Fe3O4/RGO composite. Both materials exhibit capillary condensation between 0.4 to 0.9 relative

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experimental section. High magnification FE-SEM image of RGO (Fig. 3c) exhibits the presence

stacking process. The grafting of Fe3O4 particles by RGO are shown in Fig. 3d. Fe3O4 particles are uniformly wrapped with a thin layer (4.5 nm as measured by TEM) of graphene sheet, which can improve the electronic conductivity of Fe3O4. Fig. S1 also indicating that Fe3O4 particles were uniformly wrapped with a thin layer of graphene like sheets. RGO grafted porous Fe3O4 particles may have a great impact for improved electrical performances including capacity, good rate performance, and cyclic stability. HR-TEM images of Fe3O4/RGO composites are shown in Fig. 3e and 3f. The coverage of RGO layer on Fe3O4 (indicated inside a black dot line) particles is clearly seen in Fig. 3e and 3f, which is beneficial to increase the electronic conductivity of Fe3O4 particles. The inter-planar spacing of ~ 0.48 nm (as indicated in Fig. 3e) corresponds to the (111) lattice planes of cubic Fe3O4, which is good agreement with the results of XRD analysis. The clear interface between Fe3O4 and RGO (indicated inside black dot line) is clearly seen in Fig. 3e, which acts as a linker to bind Fe3O4 particles together. The thickness of RGO layer on Fe3O4 is ~ 4.5 nm (Fig. 3f). The interlayer spacing of RGO layer in Fe3O4/RGO composite is 4.5Å (inset of Fig. 3e), which would lead a high Li-storage capacity as the interlayer spacing is large enough.46 Thus, thin layer RGO coverage is expected to increase the electro-chemical performances of Fe3O4 particles. X-ray photoelectron spectroscopy was used to study the surface chemistry of Fe3O4 and Fe3O4/RGO composite. The wide scan XPS spectrum of Fe3O4/RGO composite and Fe3O4 demonstrates the presence of (Fig. 4a and Fig. S2-a) Fe, O, and C elements. To know the chemical compositions, and the chemical oxidation states, core level spectra were recorded and the results are shown in Fig. 4b-d. From the Fig. 4b, we observe that the Fe 2p1/2 and 2p3/2 peaks 11

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of crumpled and rippled like morphology, as a result of deformation upon the exfoliation and re-

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locate at around 710.9 and 724.7 eV are broadened due to the existence of the both Fe2+ and Fe3+

literature values.48 Since the binding energy position of satellite peak of Fe 2p3/2 for different Fe oxidation states +2 or +3 occurs at 715 or 719 eV respectively, the corresponding satellite structure can be used to provide the information about the presence of FeO or γ- Fe2O3. The small hump (satellite peak) exists at ~719 eV (Fig. 4b) in the present study revealing the presence of very small amount of γ-Fe2O3. We also measured the spectrum of Fe 2p for Fe3O4 without RGO addition (Fig. S2-b, see the supporting information), and the results are similar to the conclusions derived for Fe2p of Fe3O4/RGO composite. The XPS pattern of Fe2p is in good agreement with XRD data and reveals the formation of Fe3O4 particles by microwave synthesis. In the C 1s (Fig. 4c) spectrum of Fe3O4/RGO composite, the peak at 284.6 eV corresponds to the C-C sp2 peak, whereas those at 285.5 and 288.5 eV suggest the existence of functional groups49 such as OH and C=O. The C-C peak is mainly attributed to the carbonaceous materials from reduced grapheme oxide, while C-OH, and C=O correspond to the partially dehydrated residues, which greatly change the surface properties of Fe3O4/RGO composite. The O1s spectrum (Fig. 4d) of Fe3O4/RGO composite exists at 530.1 eV revealing the presence of lattice oxygen in Fe3O4. The O1s spectrum of pure Fe3O4 (Fig. S2-c, see the supporting information) shows peaks of hydroxyl groups at 531.8 eV in addition to the lattice oxygen peak at 530.1 eV. In order to analyze the nature of carbon, Raman analysis was carried out for Fe3O4 before and after RGO addition. For comparison we also measured RGO and graphene oxide (GO) and the results are shown in Fig. 5. Raman spectrum of Fe3O4 particles (Fig. 5a) shows the absence of D and G peaks, since it doesn’t have any carbon on it. Raman spectrum of GO and RGO are presented in Fig. 5b and 5c. The two prominent peaks appeared in Fig. 5b and 5c exhibit peaks at 12

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ions47 as Fe3O4 being the mixed state of FeO and Fe2O3. These values match very well to the

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~1347 cm-1 and ~1597 cm-1, which is characteristics of ordered carbon (G) and disordered

observed from the vibration of in-plane sp2 carbon while D band relates to A1g phonon of sp3 carbon atoms of disordered graphite. A slight shift in the peaks of D and G observed between GO and RGO due to the reduction of GO. Raman spectrum of Fe3O4/RGO composite (Fig. 5d) reveals the presence of D and G band with less intensity peaks in comparison with GO and RGO. The intensity ratio of D and G (ID/IG) bands is used to evaluate the ordered and disordered crystal structures of carbon.50 The ID/IG ratio of GO, RGO and Fe3O4/RGO composite is 1.04, 1.22, and 1.52 respectively. The ID/IG ratio of GO (1.04), which is higher than graphite (0.292),51 indicating the presence of many defect sites. The increase in the ID/IG ratio of RGO (1.22) in comparison with GO (1.04) implies that synthesis process (oxidation and sonication) of RGO could lead to decrease in the sizes of in-plane sp2 domains, as well as increase in the edge planes and the disorder of GO. The ID/IG ratio of Fe3O4/RGO composite increase to 1.52 in comparison with RGO, indicating the presence of localized sp3 defects within the sp2 carbon network upon the reduction of exfoliated GO.52 The ID/IG ratio of Fe3O4/RGO composite indicates the presence of more defect or highly disordered RGO, which would be a better conductive support for Fe3O4 particles on the surface. Fig. 6 displays the TG and DSC curves of the Fe3O4/RGO composite. The weight loss occurs below 130 ˚C is due to the evaporation of water. The weight loss between 239 and 346 oC and the presence of two exothermic peaks at 262 and 300 ˚C are mainly due to the decomposition of the residual organic compounds. The majority weight loss occurs between 519 and 632 ˚C and the existence of exothermic peak at 531 ˚C is ascribed to the weight loss of carbon. The percentage of carbon in Fe3O4/RGO composite measured by TG-DSC is ~ 17 wt. %, which can be useful to calculate the specific capacity of electrode materials. The presence of 13

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carbon (D) respectively. The G band is assigned to the second order scattering of E2g mode

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exothermic peak at 803 ˚C is due to the oxidation of Fe3O4. In order to find out the existence of

400 to 4000 cm-1. Fig. 7 shows the FT-IR spectrum of as-synthesized (a) and Fe3O4/RGO composite (b). The absorption band displays at 1601cm-1 is due to the stretching vibration of C=O. The absorption bands appear at 1397 & 1342, 1216 & 992, 1121&1070, 914, 843 & 612, 533 & 489 cm-1 characteristics of various functional group including C-O-H bending vibration, C-O-C epoxy group,53 C-O stretching vibration, Fe-O-C bond,54 C-H vibration, Fe-O55 in Fe3O4 respectively. Overall FT-IR results conclude the formation of chemical bonding between Fe3O4 and graphene. 3.2 Electro-chemical performance The electro-chemical performance of Fe3O4 containing different contents of RGO (10, 30 and 50 wt. %) was carried out in the half-cell with respect to Li+ insertion/extraction mechanism. The electro-chemical experiment of Fe3O4/RGO composite was conducted in the potential range between 0.01 to 3 V at 1C. Fig. 8A shows the charge-discharge profile of the Fe3O4 composite with different RGO ratio. The reversible (second cycle discharge) capacities increase from 546 to 1739 mAh g-1, when increasing the RGO content in Fe3O4/RGO composite from 10 to 50%, and the capacities of these composites are high compared to the theoretical capacity of RGO (744 mAh g-1) and Fe3O4 (~ 924 mAh g-1). The high reversible capacities of Fe3O4/RGO composites in comparison with a theoretical capacity of Fe3O4 can be attributed to the synergistic effect between the RGO and the porous Fe3O4 particles. The high electronic conductivity of RGO can ensure good contact between Fe3O4 particles and provides an excellent conductive medium for electron as well as Li-ion conduction during electro-chemical reaction. Moreover, 14

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chemical bonding between graphene and Fe3O4, FT-IR analysis was carried out in the range from

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RGO can store Li-ions on both sides by creating LiC3 structures, which contributes additional Li-

components, the flexible RGO effectively buffers the volume change of the Fe3O4 particles, and thus preserves the structural integrity of the electrode and avoid the rapid capacity loss during electro-chemical process. Furthermore, (002) interlayer spacing of RGO in the present study is 4.5Å (inset of Fig. 3e), which would also lead to a higher Li-storage capacity since the reversible capacities can increase with expansion in the (002) interlayer spacing graphene layer, when the interlayer spacing is large enough ~4 Å.46 On the other hand, the high surface area of the Fe3O4 particles can increase the electrode–electrolyte contact area and results in a faster diffusion rate of the lithium ions. The firmer interactions between Fe3O4 particles and RGO make the lithium ions effectively and rapidly transfer back and forth from the Fe3O4 to the current collector through the highly conducting RGO network. The synergistic properties of excellent electronic conductivity of RGO and porous Fe3O4 particles with high surface area and pore volume lead to higher specific capacity and better cyclic performance compared with those of pure Fe3O4 and graphite anode materials. The cyclic performance of Fe3O4 containing different contents of RGO (10, 30 and 50 wt. %) is shown in the Fig. 8B. The first discharge capacities of 10%, 30% and 50% Fe3O4/RGO composite are 828, 2030, 2728 mAh g-1 respectively, which are higher than the theoretical capacity of Fe3O4 (~ 924 mAh g-1) attributes to the formation of the solid electrolyte interface (SEI) layer. Fe3O4 with 10% RGO shows improved cyclic performance; however the discharge capacity (242 mAh g-1) after 50 cycles is less than the theoretical capacity of Fe3O4. Though 50% Fe3O4/RGO composite shows high discharge capacities in the beginning, the capacity fading in the subsequent cycles and reached 267 mAh g-1 at the end of 50 cycles. In contrast, 30% Fe3O4/RGO composite exhibits much better performance compared to 10% and 15

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ion storage capacity of Fe3O4 particles.12,56 Besides acting as conducting bridge between different

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50%, which attributes to the presence of the optimum amount of RGO for required electrical

impact on the specific capacity. The charge-discharge profile (second cycle), and cyclic performances of Fe3O4, RGO, Fe3O4/RGO composite at 1C rate are shown in Fig. 8C and 8D respectively. Though the Fe3O4 shows high discharge capacity (1393 mAh g-1) than the capacity of Fe3O4/RGO composite (1260 mAh g-1), the large capacity fading was observed in the subsequent cycles. The discharge capacity of Fe3O4 and Fe3O4/RGO composite become 284 mAh g-1, 1030 mAh g-1 respectively after 10 cycles, indicating that Fe3O4/RGO composite retains 80% capacity retention compared to second discharge, whereas only 23% capacity retention for Fe3O4. The capacity of Fe3O4 and Fe3O4/RGO composite becomes 35 mAhg-1 and 612 mAh g-1 respectively after 50 cycles. Further, the RGO shows an irreversible capacity of 832 mAh g-1 in the initial cycle, and decreased to 202 mAh g-1 after 50 cycles. The electro-chemical stability of Fe3O4/RGO composite is much higher than bare Fe3O4 and RGO. The enhanced stability of Fe3O4/RGO composite is attributed to the synergistic effect of its individual counterpart, in which RGO addition greatly enhances the performance of Fe3O4. Fig. 9A shows chargedischarge profiles of optimized RGO contain Fe3O4 (30% Fe3O4/RGO composite) (2nd cycle) with a different current rate like 1C, 3C, and 5C. The discharge voltage profile shows a steep voltage drop (except 1C) from 2.0 to 0.85 V which can be attributed to the reaction of Li-ion insertion57 into the Fe3O4 particles as shown in equation (1). Then a long voltage plateau was observed at 0.85 V vs Li/Li+ corresponding to the conversion reaction58 (equation 2), followed by a sloping curve down to the cutoff voltage of 0.01 V, indicating the occurrence of reversible reaction59,60 (equation 3) between lithium and RGO, and the formation of solid electrolyte

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conductivity. Hence, it is believed that the amount of RGO on Fe3O4 particles has significant

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interphase (SEI) film. The voltage plateau observed for Fe3O4/RGO composite is in good

Fe 3 O 4 + xLi + xe −1 → Li x Fe 3 O 4 (1) Li x Fe 3 O 4 + (8 − x)Li + (8 − x)e −1 → 4Li 2 O (2) 2C + xLi + xe −1 → Li x C 2 (3) The rate dependent capacity performances of 30% Fe3O4/RGO composite were carried out at different current densities and shown in Fig. 9B. The first irreversible discharge capacity of 30% Fe3O4/RGO composite is 2030, 2294, and 1837 mAh g-1 for the current rate of 1 C, 3 C and 5 C respectively, which is high value in comparison with a theoretical capacity of Fe3O4 (926 mAh g1

), and might be due to the incomplete conversion reaction and irreversible lithium loss due to

the formation of SEI layer.61-63 Though, 30% Fe3O4/RGO composite (Fig. 9B) shows high irreversible discharge capacity in the first cycle, significant capacity fading was observed in the subsequent cycles. Increasing current density results significant decrease in the capacity. This could be attributed to the poor electrical contact between the electrode materials and current collector and that would decrease the capacity due to high resistance at high current rates. 30% Fe3O4/RGO composite shows reversible discharge capacity of 612, 548 and 446 mAh g-1 for 1 C, 3 C and 5 C current rate respectively after 50 cycles. 30% Fe3O4/RGO composite retains 48.17%, 42.8%, and 38.86% of the irreversible discharge capacity for 1 C, 3 C and 5 C when compared to 2nd discharge. As discussed earlier, Fe3O4 particles show high discharge capacity (1393 mAh g-1) than Fe3O4/RGO composite (1272 mAh g-1) in the beginning, the large capacity fading was observed in the subsequent cycles and the capacity becomes ~35 mAh g-1 after 50 cycles (~ 2.7% capacity retention compared to 2nd discharge cycle). This clearly indicates that the grafting of RGO into Fe3O4 particles is prerequisite to enhance the electro-chemical stability of Fe3O4. 17

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agreement with the literature reported for iron oxide/ carbon composites.

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Though initial columbic efficiency (Fig. 9B) exhibits low because of SEI formation and the

increases further to reach 98% and it maintains constant even after 50 cycles. It is interesting to note that the reversible discharge capacity of the 30% Fe3O4/RGO composite electrode is much higher than the commercial graphite anode (372 mAh g-1), and superior to carbon based Fe3O4 composite,22b,64 and graphene Fe3O4 hybrids12 reported previously. The reversible charge capacity values of Fe3O4/RGO composite are comparable with some of the Fe3O4/Graphene composites prepared by other methods.30a,65 However, the microwave combustion method having an advantage over other methods in terms of simplicity, fast reaction time, and cost-effectiveness. The electro-chemical stability of Fe3O4/RGO composite in the present study is better than the stability of Fe3O4/RGO composite reported previously.12,30a,65 Ji et al 12 reported the synthesis of Fe3O4/RGO by co-precipitation method, and showed a capacity of 389 and 334 mAh g-1 at 1C and 5C rates over 20 cycles, which is less than the capacity reported in the present study. Zang et al30a demonstrated the synthesis of magnetite/graphene composite by microwave irradiation and showed the reversible capacity of 650 and 350 mAh g-1 at 1C and 5C in the subsequent 50 cycles. Liu et al65 reported carbon encapsulated Fe3O4 nanospheres synthesized by solvothermal method, which exhibited a capacity of 636 mAh g-1 from 0.2 C to 1 C rate in the subsequent 50 cycles. Overall, the efficiency of Fe3O4/RGO composite is high and electro-chemically stable, which is better than the reported values. The superior electro-chemical performances of 30% Fe3O4/RGO composite could be explained based on the possible synergistic effect that is (i) uniform grafting of highly conducting RGO enhanced the cyclic performance of Fe3O4 due to its high electronic conductivity, (ii) the porous structure of Fe3O4 with high surface area and pore volume could shorten the diffusion paths of Li+ and electrons, and more effectively buffer the volume changes 18

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irreversible lithium loss, it increases to 94% after the first cycle. The columbic efficiency

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during Li+ insertion and extraction reactions, this argument was supported by reported

range 0 to 3.0 V at 0.1 mV.s-1 and the results are shown in Fig. 9C. It shows the oxidation and reduction peaks in the first complete scan. In the cathodic polarization of the first cycle, two obvious peaks observed at 0.85 and 0.01 V, which is attributed to the reaction of Li2O formation (Equation 2) and reversible Li-ion intercalation in RGO (Equation 3) respectively. However, 0.85 V peak slightly shifts to lower voltage 0.75 V in the second cycle, indicates that structural changes occur in the Fe3O4 particles after the Li-ion insertion in the first cycle and observed decreases in the peak intensity indicate the irreversible Li-ion loss due to the SEI formation during the first cycle.30b This peak is in good correspondence with the stable plateau observed at the same potential in the charge-discharge profile. While in the anodic polarization process, the peak observed at 1.55 eV corresponds to the oxidation of Fe0 to Fe3+. In order to understand the superior electro-chemical performances of Fe3O4/RGO composite electrode in comparison with bare Fe3O4 electrode, AC impedance measurements were performed after 3 cycles of charge/discharge process, and the results are shown in Fig. 10a. The inset shows the enlarged portion of the impedance plot of electrode materials in a certain frequency range (indicated by circle in Fig.10a). Nyquist plots (inset of Fig.10 a) showed that the diameter of the semicircle for Fe3O4/RGO composite electrode in the high-medium frequency region was much smaller than that of the bare Fe3O4 electrode, indicating that Fe3O4/RGO composite possess lower contact and charge-transfer impedances. The kinetic differences of Fe3O4/RGO composite and bare Fe3O4 electrodes were further investigated by modeling AC impendence spectra based on the modified equivalent circuit.66a,b In Fig. 10b, Re represents the internal resistance of the test battery, Rf and CPE1 are associated with the resistance and constant phase element of the SEI film, Rct and 19

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results.44,45 The cyclic voltagrams (CVs) of Fe3O4/RGO composite was conducted in the voltage

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CPE2 are associated with the charge-transfer resistance and constant phase element of the

to the lithium-diffusion process. In lithium-ion batteries, the charge transfer resistance is a measure of the charge transfer kinetics and the charge transfer process determines the rate of transfer reaction.67 It can be seen that the SEI film resistance Rf and charge-transfer resistance Rct of the Fe3O4/RGO composite electrode are 1.87 and 21.37 Ω, which are significantly lower than those of bare Fe3O4 (8.03 and 195.0 Ω). These results are in good agreement with the results of the cycle performance and rate capability, confirming that the addition of RGO increased the electronic conductivity of the composite electrode and thus greatly enhance rapid electron transport during the electrochemical lithium insertion/extraction reaction, resulting in significant improvement of the electro-chemical stability of Fe3O4/RGO composite electrode materials.

4. Conclusion In summary, porous Fe3O4/RGO composite was successfully prepared by a simple, fast, costeffective process. The structural studies revealed the presence of crystalline face centered cubic hexagonal Fe3O4 particles. The nitrogen adsorption-desorption isotherm confirms the presence of porous structure with high surface area and pore volume which shortens the diffusion paths of Li+ and electrons, and more effectively buffer the volume changes during Li+ insertion and extraction reactions. Raman spectroscopic studies of Fe3O4/RGO composite confirm the existence of graphitic carbon. The electro-chemical studies reveal that Fe3O4/RGO composite exhibit high reversible Li-ion storage capacity with enhanced cycle life and high columbic efficiency. The formation of porous Fe3O4 particles with high surface area and pore volume by microwave method, and the uniform wrapping of Fe3O4 particles by RGO could be attributed to 20

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electrode/electrolyte interface, and ZW is associated with the Warburg impedance corresponding

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the enhanced Li-ion storage of Fe3O4/RGO composite. We strongly believe that the method (a

provide a new direction for the synthesis of various electrode materials for energy storage applications. The work is underway for the efficient carbon coating on Fe3O4 to further improve the stability of electrode materials.

Acknowledgements The financial assistance received from “Nano Mission Project” of the Department of Science and Technology, Government of India is gratefully acknowledged (No.SR/NM/NS-10/2012). The authors are thankful to Prof. G. Sundararajan, Director, ARCI for his keen interest and encouragement in carrying out this work. Authors are thankful to Mr. M. Ramakrishna (Scientist, ARCI, Hyderabad) and Mrs. A. Jyothirmayi (Technical officer, ARCI, Hyderabad) for HR-TEM and impedance measurements respectively.

Electronic supporting information (ESI) available: Fig. S1, FE-SEM images of reduced graphene oxide (RGO) grafted Fe3O4; Fig. S2, XPS spectrum of Fe3O4. Survey spectrum (a), wide scan spectrum of Fe 2p (b), and O1s (c).

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61 C. Ban, Z. Wu, D. T. Gillaspie, L. Chen, Y. Yan, J. L. Blackburn and A. C. Dillon, Adv. Mater., 2010, 22, E145-E149. 62 G. Zhou, D.-W. Wang, F. Li, L. Zhang, N. Li, Z.-S. Wu, L. Wen, G. Q. Lu and H.-M. Cheng, Chem. Mater., 2010, 22, 5306-5313. 63 S. Laruelle, S. Grugeon, P. Poizot, M. Dolle, L. Dupont and J. M. Tarascon, J. Electrochem. Soc., 2002, 149, A627-A634. 64 W. F. Bin, C. Jie, H. K. Long and L. S. Qin, Sci. China Ser. E: Tech. Sci., 2009, 52, 32193223. 65 J. Liu, Y. Zhou, F. Liu, V. Liu, J. Wang, Y. Pana and D. Xue, RSC Adv., 2012, 2, 22622265. 66 (a) S. B. Yang, X. L. Feng, L. J. Zhi, Q. A. Cao, J. Maier and K. Mullen, Adv. Mater., 2010, 22, 838-842; b) S. B. Yang, H. H. Song and X. H. Chen, Electrochem. Commun., 2006, 8, 137-142. 67 G. Wang, T. Liu, Y. Luo, Y. Zhao, Z. Ren, J. Bai and H. Wang, J. Alloys Comp., 2011, 509, L216-L220.

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3909-3914.

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Figure captions:

Fig. 2 A) XRD patterns of standard Fe3O4 (a), as-synthesized Fe3O4 particles (b), Fe3O4 particles heat treated at 600˚C (c), and carbonized Fe3O4/RGO composite (d). B) Nitrogen adsorption-desorption isotherms of Fe3O4 particles (a), and Fe3O4/RGO composite (b). In order to differentiate the isotherm of Fe3O4 and Fe3O4/RGO, the isotherm value of the later has been shifted to 20 cm3g-1 (in y axis) compared with the isotherm value of the former. Fig. 3 FE-SEM images, of Fe3O4 particles (a), RGO (b, c) and Fe3O4/RGO composite (d), and HR-TEM images of Fe3O4/RGO composite (e, f).

The inset of Fig. 3a shows the

formation of porous like Fe3O4 particles. Lattice fringes of Fe3O4 and interface between Fe3O4 and RGO can be seen in Fig. 3e and 3f. The inset of Fig. 3e shows the interlayer spacing of RGO layer in Fe3O4/RGO composite. Fig. 4 XPS spectra of Fe3O4/RGO composite. Survey spectrum (a), and wide scan spectrum of Fe 2p (b), C1s (c), and O1s (d). Fig. 5 Raman spectrum of carbonized Fe3O4/RGO composite (a), RGO (b), GO (c), and assynthesized Fe3O4 particles (d). Fig. 6 Thermo gravimetric (TGA) and Differential scanning calorimetric (DSC) analysis of Fe3O4/ RGO composite. Fig. 7 FT-IR analysis of as-synthesized Fe3O4 particles (a), and Fe3O4/RGO composite (b). Fig. 8 Galvanostatic charge-discharge profiles (A) and discharge capacities (B) of Fe3O4/RGO composite with different content of RGO (10 %, 30%, and 50%); Charge-discharge profiles (C) and discharge capacities (D) of Fe3O4, RGO, and Fe3O4/RGO composite. Fig. 9 Electro-chemical performance of Fe3O4/RGO composite. Charge-discharge profile (A), cycling performance and columbic efficiency (B) of Fe3O4/RGO (30%) composite at 1 C, 3 C and 5 C current rate, and cyclic voltammetry profile(C) of Fe3O4/RGO composite at 0.1 mVS-1 scan rate. Fig. 10 Nyquist plots of Fe3O4 and Fe3O4/RGO composite electrodes (a); and equivalent circuit used for fitting (b).

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Physical Chemistry Chemical Physics Accepted Manuscript

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Fig. 1 Schematic illustration of for the synthesis process of Fe3O4/RGO composite.

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Fig. 1

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-1

Volume of nitrogen adsorbed (cm g )

3

(220)

60

20

0.0 30

0.2 40 50

0.4

30

60 (422)

(d) (c) (b) (a)

70

Relative pressure (p/p0) 0.6 0.8

80

2θ angle (degree)

120

100

(B)

80

(b)

40

(a)

0 1.0

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Fig. 2

(511)

(422)

(400)

(311)

(A) (222)

Intensity (a.u.)

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Physical Chemistry Chemical Physics Accepted Manuscript

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Fig. 3

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1200 710.9

600

710.85

400

200 600

400

200

0

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0 295

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Physical Chemistry Chemical Physics Accepted Manuscript

200

Fe2p3/2

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400

Fe 2p1/2

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726.65

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Intensity (counts)

C 1s

Fe2p

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Intensity (counts)

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1000

O1s

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Intensity (a.u.)

1000 1100 1200

Raman Shift (cm-1) 1300

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Fig. 5

G (1597)

D (1348)

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Weight loss (%)

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90 17 wt.%

Oxidation of carbon

DTA

80 200 400

Fig. 6

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Oxidation of Fe3O4

95 3

2

85 1

DSC 600 800

Temperature ( C) o 1000

0

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100

DSC (uV/mg)

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4

(a)

2000 1800 1600

-1

1400

35

-1

(b)

1200 1000

Wavenumber (cm ) -1

800 600 400

-1

-1

-1 533 cm-1 489cm

-1 612 cm

700 cm

Physical Chemistry Chemical Physics Accepted Manuscript

Fig. 7

-1 1121 cm -1 1070 cm -1 992 cm -1 914 cm -1 843 cm

1216 cm

1342 cm

1397 cm

1601 cm

-1

Transmittance (a.u.)

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2

10% 30% 50%

1

3000

(B)

10% 30% 50%

2500

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1500

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500

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0

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Fe3O4 RGO Fe3O4/RGO

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RGO Fe3O4/RGO

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Specific capacity (mAh/g)

Fe3O4

10

20

30

No. of cycles

Fig. 8

36

40

50

Physical Chemistry Chemical Physics Accepted Manuscript

Discharge capacity (mAh/g)

Potential (V vs Li/Li+)

(A)

3

0

Potential (V vs Li/Li+)

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Specific capacity (mAh/g)

Volatge (V)

2.5 2.0 1.5 1.0 0.5

1400

0

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2

1C 3C 5C

200

0.0000

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600

0.0001

0

90

1000

Specific capacity (mAh/g)

-0.0001

100

1200

0.0

Current (A)

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1C 3C 5C

3.0

110

(B)

4

Fig. 9

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50

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1600

(A)

Columbic efficiency (%)

3.5

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(a) -Zimg./ohm

40

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30 20

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100

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50 Fe3O4-RGO Fe3O4

0 0

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100

150

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250

Zreal/ohm

(b) Re

CPE1

CPE2

Rf

Rct

Fig. 10

38

Zw

300

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50

-Z img./ohm

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250

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Efficient reduced graphene oxide grafted porous Fe3O4 composites as a highperformance anode material for Li-ion batteries Subramani Bhuvaneswari, Parakandy Muzhikara Pratheeksha,a Srinivasan Anandan,* Dinesh Rangappa, Raghavan Gopalan, Tata Narasinga Rao The porous Fe3O4 particles, and the uniform RGO grafting could be attributed for the enhanced Li-ion storage of Fe3O4/RGO composite.

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Table of Content

Efficient reduced graphene oxide grafted porous Fe3O4 composite as a high performance anode material for Li-ion batteries.

Here, we report facile fabrication of Fe3O4-reduced graphene oxide (Fe3O4-RGO) composite by a novel approach, i.e., microwave assisted combustion synt...
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