Facile Preparation of Fe 3 O 4 Nanoparticles/Reduced Graphene Oxide Composite as an Efﬁcient Anode Material for Lithium-Ion Batteries

: Iron oxides are considered promising electrode materials owing to their capability of lithium storage, but their poor conductivity and large volume expansion lead to unsatisfactory cycling stability. In this paper, an inexpensive, highly effective, and facile approach to the synthesis of Fe 3 O 4 nanoparticles/reduced graphene oxide composite (Fe 3 O 4 /RGO) is designed. The synthesized Fe 3 O 4 /RGO composite exhibits high reversible capability and excellent cyclic capacity as an anode material in lithium-ion batteries (LIBs). A reversible capability of 701.8 mAh/g after 50 cycles at a current density of 200 mA · g − 1 can be maintained. The synergetic effect of unique structure and high conductivity RGO promises a well soakage of electrolyte, high structure stability, leading to an excellent electrochemical performance. It is believed that the study will provide a feasible strat-egy to produce transition metal oxide/carbon composite electrodes with excellent electrochemical performance for LIBs.


Introduction
Due to the development in electric-source vehicles as well as wearable and portable electric devices, the need to enhance the operational performance of rechargeable lithiumion batteries (LIBs) is increasing day by day [1]. In particular, the growing demand of anode materials with high reversible capacity as well as long cyclic life has increased the research interest and many papers have investigated material science and electrochemical aspects of novel materials for the above stated purpose. Among all the studied materials, TMOs (transition-metal oxides) received great attention because of their high theoretical capacity values [2][3][4][5][6][7][8]. In particular, Fe 3 O 4 is considered a prominent candidate as Li-ion battery anode because the low cost, natural abundance, environmental friendliness, and high specific capacity. However, the use of Fe 3 O 4 as LIBs anode material is responsible of low cycling capacity [9,10] due to huge voltage hysteresis during charge/discharge, large volume expansion, and low electrical conductivity during the insertion and extraction of Li-ions. Iron chloride (FeCl 3 ·6H 2 O), sulfuric acid, KMnO 4 NaOH, H 2 O 2 (3 wt.% and 30 wt.%), HCl (10 wt.%) graphite, and NaNO 3 (sodium nitrate) were obtained from Beijing Chemicals Company (Beijing, China). Without any further purification all chemicals were used as received.

Synthesis of Fe 3 O 4 /RGO Composite
GO (Graphene oxide) was synthesized according to the modified Hammer's method [21]. Firstly, a stock solution was prepared by adding NaOH (300 mg) into diethylene glycol (30 mL), heated at 130 • C under nitrogen atmosphere for 1 h. This solution cooled down to 80 • C and maintained at this temperature for further use. GO (45 mg) was added to 30 mL of diethylene glycol (DEG) under ultrasonication for 1 h, then iron chloride (180 mg) was added under vigorous stirring for 1 h. Then, under continuous nitrogen gas flow and stirring the above mixture was heated and kept at 220 • C for 0.5 h. Then, an appropriate amount of stock solution was added quickly to the reaction mixture and the temperature was maintained at 220 • C. The reaction time was fixed at 1 h to ensure the completion of reaction. Finally, ethanol was used to wash the composite Fe 3 O 4 /RGO several times for subsequent magnetic separation and keep in a vacuum oven at 60 • C for several hours. A detailed scheme of the synthesis process is given in Scheme 1.
Coatings 2021, 11, x FOR PEER REVIEW 3 of 12 sheets can also act as a high conductive substrate and fast ion transport path, which improve the electrical conductivity and ion transport in the anode material. The Fe3O4/RGO composite shows exceptional lithium storage capability, high reversible capacity, better rate performance, and stable charge/discharge cyclability (701.8 mAh/g after 50 cycles) if compared with the previous reports on Fe3O4/RGO [S12-S15].

Synthesis of Fe3O4/RGO Composite
GO (Graphene oxide) was synthesized according to the modified Hammer's method [21]. Firstly, a stock solution was prepared by adding NaOH (300 mg) into diethylene glycol (30 mL), heated at 130 °C under nitrogen atmosphere for 1 h. This solution cooled down to 80 °C and maintained at this temperature for further use. GO (45 mg) was added to 30 mL of diethylene glycol (DEG) under ultrasonication for 1 h, then iron chloride (180 mg) was added under vigorous stirring for 1 h. Then, under continuous nitrogen gas flow and stirring the above mixture was heated and kept at 220 °C for 0.5 h. Then, an appropriate amount of stock solution was added quickly to the reaction mixture and the temperature was maintained at 220 °C. The reaction time was fixed at 1 h to ensure the completion of reaction. Finally, ethanol was used to wash the composite Fe3O4/RGO several times for subsequent magnetic separation and keep in a vacuum oven at 60 °C for several hours. A detailed scheme of the synthesis process is given in Scheme 1.

Scheme 1.
Scheme of the synthesis process of Fe3O4/RGO composite.

Characterization
Scanning electron microscopy (JEOL JEM-6701F, Peabody, MA, USA) was utilized for surface morphological observations. X-ray diffraction (XRD: D8 system with CuKα = 1.55, motorized divergence slit with monochromator, high intensity Ka 1, 2 parallel beam, 2θ = 4°-90°, Tokyo, Japan Instrument) was employed for crystal structure analysis. TEM (transmission electron microscopy) was performed using a JEOL JEM-3010F (Peabody, MA, USA) model and XPS (X-ray photoelectron spectroscopy) using a K-Alpha Thermo Fisher Scientific, Waltham, MA, USA) to measure the atomic concentrations of composite surface and chemical binding energies. Surface area measurements are carried out by nitrogen adsorption-desorption at 77 K on a micromeritics Asap 2460 using the BET Scheme 1. Scheme of the synthesis process of Fe 3 O 4 /RGO composite.

Characterization
Scanning electron microscopy (JEOL JEM-6701F, Peabody, MA, USA) was utilized for surface morphological observations. X-ray diffraction (XRD: D8 system with CuKα = 1.55, motorized divergence slit with monochromator, high intensity Ka 1, 2 parallel beam, 2θ = 4 • -90 • , Tokyo, Japan Instrument) was employed for crystal structure analysis. TEM (transmission electron microscopy) was performed using a JEOL JEM-3010F (Peabody, MA, USA) model and XPS (X-ray photoelectron spectroscopy) using a K-Alpha Thermo Fisher Scientific, Waltham, MA, USA) to measure the atomic concentrations of composite surface and chemical binding energies. Surface area measurements are carried out by nitrogen adsorption-desorption at 77 K on a micromeritics Asap 2460 using the BET (Brunauer-Emmett-Teller) theory. To check the thermal stability of composite, thermogravimetric analysis (TA SDT Q600 instrument, New Castle, DE, USA) was performed under N 2 /atm at a heating rate of 10 • C·min −1 from 25 to 800 • C. Degree and type of defects in the composite were determined by Raman spectroscopy (Renishaw RM1000 confocal microscope, R&R Sales and Engineering, LLC, Renishaw GmbH, Pliezhausen, Germany).

Electrochemical Measurements
The electrochemical properties of the as-synthesized composite are studied by using a coin type cell. The assembly of cell is performed in a glove box in Ar atmosphere. The as-prepared composite material was used as the working electrode materials. The slurry coating method was used for the preparation of the working electrode with a composition of 70 wt.% of composite, 20 wt.% of carbon black, and 10 wt.% ploy (vinylidene difluoride) in N-methyl pyrrolidinone. After that, the slurry was homogenously casted onto nickel foam and vacuum dried at 140 • C for 20 h. Metallic lithium foil was used as counter electrode 1 M LiPF 6 was used as electrolyte in a mixture of ethyl carbonate/diethyl carbonate (EC/DEC) with a volumetric ratio of 1:1. The charge/discharge profiles test were taken in the voltage range of 0.02-3.0 V at a constant current density of 200 mA/g. Ciclovoltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) measurements at the as-prepared anode material were taken using a workstation (CHI 660B). The CV measurements were employed within the range of 0.02-3.0 V at a constant rate of 0.1 mV·s −1 . A frequency range from 100 kHz to 10 MHz was used to perform the EIS measurements.

Results
XRD patterns of the prepared GO and Fe 3 O 4 /RGO nanocomposite are reported in Figure 1. The GO pattern shows two peaks positioned around 2θ = 10.9 • and 42.6 • , which are characteristic XRD diffraction peaks of GO. Introduction of different oxygen functionalities (such as epoxy carbonyl, hydroxyl) appearing at the surface of GO sheet due to strong oxidization are evidenced by the peak at 10.9 • whereas the peak centered at 42.6 • is characteristic of (100) plane of hexagonal structure of carbon [39,40]. The diffraction peaks shown by Fe 3 O 4 /RGO composite are characteristic of the magnetite face-centered cubic structure (JCPDS card No. 19-0629). An additional (002) diffraction peak around 25 • in Fe 3 O 4 /RGO composite can be attributed to less agglomerated and disordered sheets of RGO.
(Brunauer-Emmett-Teller) theory. To check the thermal stability of composite, thermogravimetric analysis (TA SDT Q600 instrument, New Castle, DE, USA) was performed under N2/atm at a heating rate of 10 °C·min −1 from 25 to 800 °C. Degree and type of defects in the composite were determined by Raman spectroscopy (Renishaw RM1000 confocal microscope, R&R Sales and Engineering, LLC, Renishaw GmbH, Germany).

Electrochemical Measurements
The electrochemical properties of the as-synthesized composite are studied by using a coin type cell. The assembly of cell is performed in a glove box in Ar atmosphere. The as-prepared composite material was used as the working electrode materials. The slurry coating method was used for the preparation of the working electrode with a composition of 70 wt.% of composite, 20 wt.% of carbon black, and 10 wt.% ploy (vinylidene difluoride) in N-methyl pyrrolidinone. After that, the slurry was homogenously casted onto nickel foam and vacuum dried at 140 °C for 20 h. Metallic lithium foil was used as counter electrode 1 M LiPF6 was used as electrolyte in a mixture of ethyl carbonate/diethyl carbonate (EC/DEC) with a volumetric ratio of 1:1. The charge/discharge profiles test were taken in the voltage range of 0.02-3.0 V at a constant current density of 200 mA/g. Ciclovoltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) measurements at the as-prepared anode material were taken using a workstation (CHI 660B). The CV measurements were employed within the range of 0.02-3.0 V at a constant rate of 0.1 mV·s −1 . A frequency range from 100 kHz to 10 MHz was used to perform the EIS measurements.

Results
XRD patterns of the prepared GO and Fe3O4/RGO nanocomposite are reported in Figure 1. The GO pattern shows two peaks positioned around 2θ = 10.9° and 42.6°, which are characteristic XRD diffraction peaks of GO. Introduction of different oxygen functionalities (such as epoxy carbonyl, hydroxyl) appearing at the surface of GO sheet due to strong oxidization are evidenced by the peak at 10.9° whereas the peak centered at 42.6° is characteristic of (100) plane of hexagonal structure of carbon [39,40]. The diffraction peaks shown by Fe3O4/RGO composite are characteristic of the magnetite face-centered cubic structure (JCPDS card No. 19-0629). An additional (002) diffraction peak around 25° in Fe3O4/RGO composite can be attributed to less agglomerated and disordered sheets of RGO.   Figure 2a. The sample was heated under nitrogen atmosphere from 20 to 800 • C at 10 • C·min −1 . The initial weight loss observed from room temperature to 150 • C in the curve relating to GO is due to the evaporation of adsorbed water. Approximately at 300 • C, the substantial increase in weight loss is attributed to the removal of various oxygen containing functionalities. The subsequent decrease from 350 • C onwards to 800 • C can be ascribed to the pyrolysis of carbon skeleton as well as to the elimination of more thermal stable oxygen functionalities. In contrast, the TGA curve of Fe 3 O 4 /RGO composite shows a greatly constrained weight loss. This phenomenon indicated that Fe 3 O 4 NPs established in the composite a strong interaction with RGO layers. This caused a restriction effect on movement of RGO sheets which avoid heat concentration and thus helped in homogeneous heating of composite [41]. Raman spectra of GO and Fe 3 O 4 /RGO composite are shown in Figure 2b. Raman is a very useful technique to study the nature of pristine and composite carbon materials giving insight in the structural feathers before and after composite formation. Raman spectra of GO showed two peaks around~1349 and~1599 cm −1 characteristic of disordered carbon D band and ordered carbon G bands, respectively. The D band peak is due to the sp 3 carbon atoms of disordered graphite and the peak relating to G band can be ascribed to the in-plane vibrations of sp 2 carbon atoms. Fe 3 O 4 /RGO composite Raman spectrum also shows both D and G bands as prominent peaks, but the G band is slightly shifted towards lower wavenumber from 1599-1589 cm −1 , giving an indication of the reduction of GO. In case of GO, D band peak is higher as compared to G band peak while in case of Fe 3 O 4 /RGO D band peaks is lower as compared to G band peak. The I D /I G ratio is another feature of the Raman spectrum, which gives useful information about the reduction of GO; in case of reduction the value of I D /I G ratios increases. For the Fe 3 O 4 /RGO composite, the I D /I G ratio is estimated to be 1.83 while for GO a value of 1.76 is calculated. This increase in intensity ratios indicates more defects and high degree of disorderliness (presence of localized sp 3 defects within the sp 2 carbon skeleton [42]). at 10 °C·min −1 . The initial weight loss observed from room temperature to 150 °C in the curve relating to GO is due to the evaporation of adsorbed water. Approximately at 300 °C, the substantial increase in weight loss is attributed to the removal of various oxygen containing functionalities. The subsequent decrease from 350 °C onwards to 800 °C can be ascribed to the pyrolysis of carbon skeleton as well as to the elimination of more thermal stable oxygen functionalities. In contrast, the TGA curve of Fe3O4/RGO composite shows a greatly constrained weight loss. This phenomenon indicated that Fe3O4 NPs established in the composite a strong interaction with RGO layers. This caused a restriction effect on movement of RGO sheets which avoid heat concentration and thus helped in homogeneous heating of composite [41]. Raman spectra of GO and Fe3O4/RGO composite are shown in Figure 2b. Raman is a very useful technique to study the nature of pristine and composite carbon materials giving insight in the structural feathers before and after composite formation. Raman spectra of GO showed two peaks around ~1349 and ~1599 cm −1 characteristic of disordered carbon D band and ordered carbon G bands, respectively. The D band peak is due to the sp 3 carbon atoms of disordered graphite and the peak relating to G band can be ascribed to the in-plane vibrations of sp 2 carbon atoms. Fe3O4/RGO composite Raman spectrum also shows both D and G bands as prominent peaks, but the G band is slightly shifted towards lower wavenumber from ~1599-1589 cm −1 , giving an indication of the reduction of GO. In case of GO, D band peak is higher as compared to G band peak while in case of Fe3O4/RGO D band peaks is lower as compared to G band peak. The ID/IG ratio is another feature of the Raman spectrum, which gives useful information about the reduction of GO; in case of reduction the value of ID/IG ratios increases. For the Fe3O4/RGO composite, the ID/IG ratio is estimated to be 1.83 while for GO a value of 1.76 is calculated. This increase in intensity ratios indicates more defects and high degree of disorderliness (presence of localized sp 3 defects within the sp 2 carbon skeleton [42]).  Figure 3a shows the XPS survey spectrum in the energy range of 0-1300 eV of GO and Fe3O4/RGO composite material. Fe, O, and C elements are detected for Fe3O4/RGO composite sample and C and O elements for GO sample as expected. By using XPS peak fitting software, C 1s spectra of GO could be resolved into four peaks, which were situated around 288.8, 287.8, 286.7, and 284.5 eV. In the case of GO, the atomic percentages were found to be 59.4 and 40.6 for carbon and oxygen, respectively, corresponding to an O/C atomic ratio of 0.68. C 1s spectra relating to Fe3O4/RGO composite shows a decrease in peak lie at 286.7 suggesting the decrease in C-O and C=O functional groups in the composite, while all the other peaks are similar to the peaks recorded in the C 1s spectra of GO. Figure 3d showed the Fe 2p high resolution XPS spectrum of Fe3O4/RGO composite.  for GO sample as expected. By using XPS peak fitting software, C 1s spectra of GO could be resolved into four peaks, which were situated around 288.8, 287.8, 286.7, and 284.5 eV. In the case of GO, the atomic percentages were found to be 59.4 and 40.6 for carbon and oxygen, respectively, corresponding to an O/C atomic ratio of 0.68. C 1s spectra relating to Fe 3 O 4 /RGO composite shows a decrease in peak lie at 286.7 suggesting the decrease in C-O and C=O functional groups in the composite, while all the other peaks are similar to the peaks recorded in the C 1s spectra of GO. Figure 3d showed the Fe 2p high resolution XPS spectrum of Fe 3 O 4 /RGO composite. The peaks at 711.7 and 725.3 eV correspond to the Fe 2p3/2 and Fe 2p1/2 orbitals, respectively, in agreement with previously published values for Fe 3 O 4 [43]. The absence of any peak around 720 eV confirms that no charge transfer satellite of Fe 2p3/2 was present, and thus the formation of mixed oxides of Fe +3 and Fe +2 can be excluded [44]. The peaks at 711.7 and 725.3 eV correspond to the Fe 2p3/2 and Fe 2p1/2 orbitals, respectively, in agreement with previously published values for Fe3O4 [43]. The absence of any peak around 720 eV confirms that no charge transfer satellite of Fe 2p3/2 was present, and thus the formation of mixed oxides of Fe +3 and Fe +2 can be excluded [44]. SEM analysis reported in (Figure 4a,b), shows that Fe3O4/RGO composite is formed by RGO individual sheets densely covered by uniform sized Fe3O4 NPs without any visible and significant agglomerations. These dense coverings of ultra-fine Fe3O4 NPs on both the sides of RGO layers not only increase thermal stability, as revealed by TGA analysis, but it is also expected to improve the conductivity of composite. The reduction of GO partially restores the π electronic system, which can cause prominent restacking of reduced graphene layers due to π-π interactions. This uniform covering of Fe3O4 NPs also makes difficult the restacking of reduced graphene layers of GO because reduction of GO and Fe 3+ ions to Fe3O4 NPs occurs simultaneously in one step. No visible uncovered RGO is found in SEM pictures. At higher magnifications (Figure 4 c,d) one can see the sheet structure of RGO in the composite. This morphology is believed to provide an easy path for Li ions diffusion and to accommodate the volume changes effect adding up the specific capacity of the LIB. Energy-dispersive X-ray spectroscopy (EDX) element mapping was employed to estimate the iron oxide nanoparticles in the composite and the uniformity of their dispersion in the RGO substrate ( Figure S1 Supplementary Information). EDX analysis revealed the excellent uniform dispersion of iron oxide nanoparticles with a Fe weight content of 23.94 wt.%. TEM images of RGO/Fe3O4 NPs composite are shown in Figure 4 e,f. The presence of large amounts of oxygenated functionalities, such as epoxy, carboxyl, and hydroxyl, at surface of GO sheets help to anchor hydroxylated iron complexes onto the surfaces of GO sheets. Fe3O4 NPs are grown on GO sheets to form surface-bound oxygen bridges nanocomposites at elevated temperature in alkaline media with the addition of NaOH. Figure 4e,f shows the tight bond between the components in agreement with SEM analysis reported in (Figure 4a,b), shows that Fe 3 O 4 /RGO composite is formed by RGO individual sheets densely covered by uniform sized Fe 3 O 4 NPs without any visible and significant agglomerations. These dense coverings of ultra-fine Fe 3 O 4 NPs on both the sides of RGO layers not only increase thermal stability, as revealed by TGA analysis, but it is also expected to improve the conductivity of composite. The reduction of GO partially restores the π electronic system, which can cause prominent restacking of reduced graphene layers due to π-π interactions. This uniform covering of Fe 3 O 4 NPs also makes difficult the restacking of reduced graphene layers of GO because reduction of GO and Fe 3+ ions to Fe 3 O 4 NPs occurs simultaneously in one step. No visible uncovered RGO is found in SEM pictures. At higher magnifications (Figure 4 c,d) one can see the sheet structure of RGO in the composite. This morphology is believed to provide an easy path for Li ions diffusion and to accommodate the volume changes effect adding up the specific capacity of the LIB. Energy-dispersive X-ray spectroscopy (EDX) element mapping was employed to estimate the iron oxide nanoparticles in the composite and the uniformity of their dispersion in the RGO substrate ( Figure  TGA and XPS results. TEM also revealed that individual RGO sheets are densely and homogeneously covered with Fe3O4 NPs of uniform diameter. TEM images (e,f) further support the SEM findings. No obvious uncovered portions as well as no considerable odd size particles can be found. N2 absorption/desorption isotherm has been carried out to measure the surface area and porosity of as prepared composite and reported in Figure 5a,b. The surface area of the composite is measured by BET as 103.29 m 2 /g. The value is quite high and gives an indication about the presence of active and functional sites. The Barrett, Joyner, and Halenda desorption gives a pore volume of 0.209235 cm 3 /g. Highly dispersed RGO contributes to the high value of surface area of the Fe3O4/RGO composite, which in turn offer easy diffusion of the electrolyte and a large number of active sites [45]. The hysteretic curve of Fe3O4/RGO is a type "IV" isotherm characteristic of mesoporous materials. The high surface area can make easy and rapid insertion/extraction of Li + ions because of presence of additional channels and active sites in composite anode material. The sheet structure of the composite not only buffers the volume changes during charging/discharging steps, but also assists diffusion of solid-state Li + ions to maintain the overall integrity of the structure. PSD curves shows a maximum distribution of the pores between 0.5 to 6 nm.  N 2 absorption/desorption isotherm has been carried out to measure the surface area and porosity of as prepared composite and reported in Figure 5a,b. The surface area of the composite is measured by BET as 103.29 m 2 /g. The value is quite high and gives an indication about the presence of active and functional sites. The Barrett, Joyner, and Halenda desorption gives a pore volume of 0.209235 cm 3 /g. Highly dispersed RGO contributes to the high value of surface area of the Fe 3 O 4 /RGO composite, which in turn offer easy diffusion of the electrolyte and a large number of active sites [45]. The hysteretic curve of Fe 3 O 4 /RGO is a type "IV" isotherm characteristic of mesoporous materials. The high surface area can make easy and rapid insertion/extraction of Li + ions because of presence of additional channels and active sites in composite anode material. The sheet structure of the composite not only buffers the volume changes during charging/discharging steps, but also assists diffusion of solid-state Li + ions to maintain the overall integrity of the structure. PSD curves shows a maximum distribution of the pores between 0.5 to 6 nm.  N2 absorption/desorption isotherm has been carried out to measure the surface area and porosity of as prepared composite and reported in Figure 5a,b. The surface area of the composite is measured by BET as 103.29 m 2 /g. The value is quite high and gives an indication about the presence of active and functional sites. The Barrett, Joyner, and Halenda desorption gives a pore volume of 0.209235 cm 3 /g. Highly dispersed RGO contributes to the high value of surface area of the Fe3O4/RGO composite, which in turn offer easy diffusion of the electrolyte and a large number of active sites [45]. The hysteretic curve of Fe3O4/RGO is a type "IV" isotherm characteristic of mesoporous materials. The high surface area can make easy and rapid insertion/extraction of Li + ions because of presence of additional channels and active sites in composite anode material. The sheet structure of the composite not only buffers the volume changes during charging/discharging steps, but also assists diffusion of solid-state Li + ions to maintain the overall integrity of the structure. PSD curves shows a maximum distribution of the pores between 0.5 to 6 nm.  The discharge/charge capacities are initially 1425.7 and 880.8 mAh/g, respectively. The capacity loss observed at the first cycle for most of the samples is about 62% due to the irreversible Solid Electrolyte Interphase (SEI) formation. During the first discharge step, at about 0.8 V a long voltage plateau tailed by a sloping curve down to the cut voltage of 0.01 V was observed for all the recorded curves. This behavior depends on the features of voltage drifts characteristic of the Fe3O4 electrode [46,47]. It is well-known that the sheet type structure of Fe3O4/RGO may trap more lithium ions during electrochemical cycling, which resulted in large irreversible capacities. The reversibility of the composite electrode is significantly improved in the 2nd cycle. After the 3rd cycle, the discharge/charge capacities are 871.3 and 816 mAh/g, respectively, and the columbic efficiency exceeds 94% signifying a better capacity retention of the electrode. The slight capacity change in the successive cycles is attributed to the maximum probability of finding favorable sites for Li + ion migration in the nanoscaled Fe3O4 particles. Consequently, the increase in reversible capacities could be ascribed to the fast lithium ion and transport of electron through covalent interaction between the Fe3O4 nanoparticles and RGO. Moreover, the Fe3O4 NPs can enable the infiltration of electrolyte which is responsible to speed up the lithium-ion transport. Figure 6b shows the cyclic performance of as-synthesized Fe3O4/RGO composite. Fe3O4/RGO nanocomposite electrodes exhibit better reversible capacity at a current density of 200 mA/g after 50 cycles. The retentive capacity (701.8 mAh/g) after 50 cycles is 83% of the second reversible charge capacity (850.1 mAh/g) indicating the longer cycling life and excellent strength of the electrode. Throughout successive electrochemical cycling (at about the 15th cycle) a fading of capacity is observed in all samples and attributed to the mechanical damage/severe fracture or to the agglomeration of Fe3O4 NPs owing to the drastic volume changes along with the conversion reaction. Nevertheless, the Fe3O4/RGO composite investigated in this work show better rate performance and stable charge/discharge cyclability (701.8 mAh/g after 50 cycles) if compared with the previous reports on Fe3O4/RGO and similar Fe-containing composites. A comprehensive comparison between the electrochemical performances of the Fe3O4/RGO composites studied in this paper and The discharge/charge capacities are initially 1425.7 and 880.8 mAh/g, respectively. The capacity loss observed at the first cycle for most of the samples is about 62% due to the irreversible Solid Electrolyte Interphase (SEI) formation. During the first discharge step, at about 0.8 V a long voltage plateau tailed by a sloping curve down to the cut voltage of 0.01 V was observed for all the recorded curves. This behavior depends on the features of voltage drifts characteristic of the Fe 3 O 4 electrode [46,47]. It is well-known that the sheet type structure of Fe 3 O 4 /RGO may trap more lithium ions during electrochemical cycling, which resulted in large irreversible capacities. The reversibility of the composite electrode is significantly improved in the 2nd cycle. After the 3rd cycle, the discharge/charge capacities are 871.3 and 816 mAh/g, respectively, and the columbic efficiency exceeds 94% signifying a better capacity retention of the electrode. The slight capacity change in the successive cycles is attributed to the maximum probability of finding favorable sites for Li + ion migration in the nanoscaled Fe 3 O 4 particles. Consequently, the increase in reversible capacities could be ascribed to the fast lithium ion and transport of electron through covalent interaction between the Fe 3 O 4 nanoparticles and RGO. Moreover, the Fe 3 O 4 NPs can enable the infiltration of electrolyte which is responsible to speed up the lithium-ion transport. Figure 6b shows the cyclic performance of as-synthesized Fe 3 O 4 /RGO composite.  Table S1 and references therein (Supporting information). This improvement is attributed to the homogenous decoration of Fe 3 O 4 NPs in the layered graphene observed in the Fe 3 O 4 /RGO composite electrode by SEM and TEM analysis and confirmed by TGA measurements. Such a dimensional confinement of the Fe 3 O 4 NPs by the surrounding graphene layers reduces the volume increase effect upon insertion of lithium. As a consequence, graphene layers provide a conductive network favorable to the electron transfer during lithiation/delithiation process.
The electrode rate capabilities were also evaluated at various current densities as depicted in Figure 6c. The differential plots of capacity vs. cell voltage presents in Figure 6d shows the lithium storage capability of Fe 3 O 4 /RGO is mainly at the voltage of capacity 0.5 V.
As the current density increases from 200 to 1500 mA/g, the decrease in discharge capacity is attributed to the controlled-diffusion kinetics process for the electrode reaction. After 10 cycles the discharge capacity noted for Fe 3 O 4 /RGO composite is 874.1 mAh/g at a current density of 200 mA/g. The discharge capacities of Fe 3 O 4 /RGO still remain at 740.8, 578.1, and 452.4 mAh/g even at a very high current density of 500, 1000, and 1500 mA/g, respectively. Furthermore, an excellent reversibility was obtained after deep cycling at 1500 mA/g; when the current density brings back to 200 mA/g, the discharge capacity still retains at 875.3 mAh/g. According to our knowledge, the Fe 3 O 4 /RGO composite synthesized by using the method described in this paper possess an improved rate retention property if used as anode in an LIB. A comparison between carbon-based Fe 3 O 4 composite anodes for LIBs reported in previous studies is given in Table S1 (supplementary  information). Structure stability and high electrochemical activity is answerable for the improved electrochemical performance of the composite material.
The reversible reaction of Li ions with as synthesized Fe 3 O 4 /RGO composite was explored in view of practical application as anode material in Li-ion batteries. Figure 7 shows the CV curves (first three cycles) of Fe 3 O 4 /RGO composite electrode at a scan rate of 0.1 mV·s −1 .
Coatings 2021, 11, x FOR PEER REVIEW 9 of 12 the Fe3O4/graphene and Fe3O4/carbon materials previously reported in literature is shown in Table S1 and references therein (Supporting information). This improvement is attributed to the homogenous decoration of Fe3O4 NPs in the layered graphene observed in the Fe3O4/RGO composite electrode by SEM and TEM analysis and confirmed by TGA measurements. Such a dimensional confinement of the Fe3O4 NPs by the surrounding graphene layers reduces the volume increase effect upon insertion of lithium. As a consequence, graphene layers provide a conductive network favorable to the electron transfer during lithiation/delithiation process. The electrode rate capabilities were also evaluated at various current densities as depicted in Figure 6c. The differential plots of capacity vs. cell voltage presents in Figure 6d shows the lithium storage capability of Fe3O4/RGO is mainly at the voltage of capacity 0.5 V.
As the current density increases from 200 to 1500 mA/g, the decrease in discharge capacity is attributed to the controlled-diffusion kinetics process for the electrode reaction. After 10 cycles the discharge capacity noted for Fe3O4/RGO composite is 874.1 mAh/g at a current density of 200 mA/g. The discharge capacities of Fe3O4/RGO still remain at 740.8, 578.1, and 452.4 mAh/g even at a very high current density of 500, 1000, and 1500 mA/g, respectively. Furthermore, an excellent reversibility was obtained after deep cycling at 1500 mA/g; when the current density brings back to 200 mA/g, the discharge capacity still retains at 875.3 mAh/g. According to our knowledge, the Fe3O4/RGO composite synthesized by using the method described in this paper possess an improved rate retention property if used as anode in an LIB. A comparison between carbon-based Fe3O4 composite anodes for LIBs reported in previous studies is given in Table S1 (supplementary information). Structure stability and high electrochemical activity is answerable for the improved electrochemical performance of the composite material.
The reversible reaction of Li ions with as synthesized Fe3O4/RGO composite was explored in view of practical application as anode material in Li-ion batteries. Figure 7 shows the CV curves (first three cycles) of Fe3O4/RGO composite electrode at a scan rate of 0.1 mV·s −1 . During the first cathodic scan of Fe3O4/RGO composite, the sharp reduction peak observed at about 0.7 V is ascribed to the irreversible reaction with electrolyte and to the reduction to the different oxidation states of iron (Fe 3+ or Fe 2+ to Fe 0 ) [17]. Furthermore, the two reduction peaks of less intensity at 0.9 and 1.5 V can be attributed to the formation of LixFe3O4 [48,49]. During the first discharge potential scan, the conversion of Fe3O4 to Fe and formation of Li2O could be the key reason for irreversible capacity.
In the meantime, the reversible oxidation of Fe 0 to Fe 2+ /Fe 3+ is evidenced by the two anodic peaks at 1.63 and 1.85 V [46]. The shift of the redox peaks in succeeding cycles occurs owing to structural modifications. It is worth noting that the area underlying the During the first cathodic scan of Fe 3 O 4 /RGO composite, the sharp reduction peak observed at about 0.7 V is ascribed to the irreversible reaction with electrolyte and to the reduction to the different oxidation states of iron (Fe 3+ or Fe 2+ to Fe 0 ) [17]. Furthermore, the two reduction peaks of less intensity at 0.9 and 1.5 V can be attributed to the formation of Li x Fe 3 O 4 [48,49]. During the first discharge potential scan, the conversion of Fe 3 O 4 to Fe and formation of Li 2 O could be the key reason for irreversible capacity.
In the meantime, the reversible oxidation of Fe 0 to Fe 2+ /Fe 3+ is evidenced by the two anodic peaks at 1.63 and 1.85 V [46]. The shift of the redox peaks in succeeding cycles occurs owing to structural modifications. It is worth noting that the area underlying the peak at 1.85 V is larger for second and third cycle accounting for the reversible reaction of Fe 3 O 4 . Moreover, after the first cycle no obvious changes were found in peak intensity as well as in integrated area of both anodic and cathodic peaks, signifying that the electrochemical reversibility of Fe 3 O 4 -graphene become stable after the initial cycle. The CV results are in good agreement with the charge/discharge results.
To further verify the improved electrochemical properties of the as-synthesized composite, EIS measurements were employed. The Nyquist plot of Fe 3 O 4 /RGO composite after five cycles at a current density of 200 mA/g are shown in Figure 8. From the plot it can be qualitatively observed that the diameter of the semicircle relating to the Fe 3 O 4 /RGO composite is small (24 Coatings 2021, 11, x FOR PEER REVIEW 10 of 12 peak at 1.85 V is larger for second and third cycle accounting for the reversible reaction of Fe3O4. Moreover, after the first cycle no obvious changes were found in peak intensity as well as in integrated area of both anodic and cathodic peaks, signifying that the electrochemical reversibility of Fe3O4-graphene become stable after the initial cycle. The CV results are in good agreement with the charge/discharge results. To further verify the improved electrochemical properties of the as-synthesized composite, EIS measurements were employed. The Nyquist plot of Fe3O4/RGO composite after five cycles at a current density of 200 mA/g are shown in Figure 8. From the plot it can be qualitatively observed that the diameter of the semicircle relating to the Fe3O4/RGO composite is small (24 Ώ ) indicating that the Fe3O4/RGO composite electrode considerably lowers the contact and charge transfer resistances thanks to the high content of carbon and layered structure of graphene [50]. This result corroborates with XPS, TGA and Raman results indicating that a strong contact between Fe3O4 and RGO sheets can effectively increase the charge transfer and reduce the contact resistances.

Conclusions
A simple and facile approach was developed to obtain Fe3O4/RGO composite in which Fe3O4 NPs homogenously cover the surface of RGO sheets. The improved electrochemical performance was attributed to the synergistic effect between Fe3O4 NPs and RGO. The Fe3O4/RGO composite exhibited an outstanding specific capacity up to 701.8 mAh/g after 50 cycles, even at a high current density of 1500 mA/g. The composite possesses considerable higher capacity (446.4 mAh/g) owing to the sheet structural symmetry. The graphene sheets can effectively reduce the lithium diffusion paths offering better conductivity, as well as provide large contact area for the Fe3O4 NPs. The results discussed in this paper allow to conclude that the Fe3O4/RGO composite represents a promising anode material for LIBs.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: EDX and corresponding elemental mapping of C, O, and Fe, Table S1: Capacity of carbon based Fe3O4 anode materials for LIBS. ) indicating that the Fe 3 O 4 /RGO composite electrode considerably lowers the contact and charge transfer resistances thanks to the high content of carbon and layered structure of graphene [50].
Coatings 2021, 11, x FOR PEER REVIEW 10 of 12 peak at 1.85 V is larger for second and third cycle accounting for the reversible reaction of Fe3O4. Moreover, after the first cycle no obvious changes were found in peak intensity as well as in integrated area of both anodic and cathodic peaks, signifying that the electrochemical reversibility of Fe3O4-graphene become stable after the initial cycle. The CV results are in good agreement with the charge/discharge results.
To further verify the improved electrochemical properties of the as-synthesized composite, EIS measurements were employed. The Nyquist plot of Fe3O4/RGO composite after five cycles at a current density of 200 mA/g are shown in Figure 8. From the plot it can be qualitatively observed that the diameter of the semicircle relating to the Fe3O4/RGO composite is small (24 Ώ) indicating that the Fe3O4/RGO composite electrode considerably lowers the contact and charge transfer resistances thanks to the high content of carbon and layered structure of graphene [50]. This result corroborates with XPS, TGA and Raman results indicating that a strong contact between Fe3O4 and RGO sheets can effectively increase the charge transfer and reduce the contact resistances.

Conclusions
A simple and facile approach was developed to obtain Fe3O4/RGO composite in which Fe3O4 NPs homogenously cover the surface of RGO sheets. The improved electrochemical performance was attributed to the synergistic effect between Fe3O4 NPs and RGO. The Fe3O4/RGO composite exhibited an outstanding specific capacity up to 701.8 mAh/g after 50 cycles, even at a high current density of 1500 mA/g. The composite possesses considerable higher capacity (446.4 mAh/g) owing to the sheet structural symmetry. The graphene sheets can effectively reduce the lithium diffusion paths offering better conductivity, as well as provide large contact area for the Fe3O4 NPs. The results discussed in this paper allow to conclude that the Fe3O4/RGO composite represents a promising anode material for LIBs.

Supplementary Materials:
The following are available online at www.mdpi.com/xxx/s1, Figure S1: EDX and corresponding elemental mapping of C, O, and Fe, Table S1: Capacity of carbon based Fe3O4 anode materials for LIBS.  This result corroborates with XPS, TGA and Raman results indicating that a strong contact between Fe 3 O 4 and RGO sheets can effectively increase the charge transfer and reduce the contact resistances.

Conclusions
A simple and facile approach was developed to obtain Fe 3 O 4 /RGO composite in which Fe 3 O 4 NPs homogenously cover the surface of RGO sheets. The improved electrochemical performance was attributed to the synergistic effect between Fe 3 O 4 NPs and RGO. The Fe 3 O 4 /RGO composite exhibited an outstanding specific capacity up to 701.8 mAh/g after 50 cycles, even at a high current density of 1500 mA/g. The composite possesses considerable higher capacity (446.4 mAh/g) owing to the sheet structural symmetry. The graphene sheets can effectively reduce the lithium diffusion paths offering better conductivity, as well as provide large contact area for the Fe 3 O 4 NPs. The results discussed in this paper allow to conclude that the Fe 3 O 4 /RGO composite represents a promising anode material for LIBs.