Graphene nanosheets loaded Fe3O4 nanoparticles as a promising anode material for lithium ion batteries

Graphene nanosheets loaded Fe3O4 nanoparticles as a promising anode material for lithium ion batteries

Journal of Alloys and Compounds 813 (2020) 152160 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:/...

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Journal of Alloys and Compounds 813 (2020) 152160

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Graphene nanosheets loaded Fe3O4 nanoparticles as a promising anode material for lithium ion batteries Siling Gu, Aiping Zhu* School of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu, 225002, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2019 Received in revised form 29 August 2019 Accepted 4 September 2019 Available online 4 September 2019

A novel synthesis strategy has been developed to synthesize graphene nanosheets loaded Fe3O4 nanoparticles (GNSs/Fe3O4 NPs) via a freeze-drying of graphene and iron ion suspension and by a solvent thermal synthesis method sequentially. The size of Fe3O4 loaded on the graphene nanosheets can be regulated from 20 nm to 100 nm by controlling the mass ratio of GNSs and Fe3O4. When the ratio of GNSs to Fe3O4 is 1:2, the obtained Fe3O4 nanoparticles have ~40 nm in diameter. Lithium ion battery anode made from GNSs/Fe3O4 (1:2) without adding any other conductive agent can exhibit a high reversible capacity (~1145 mAh g1 after 120 cycles at 100 mA g1) and a remarkable rate capability (650 and 530 mAh g1 at 0.5 and 1 A g1, respectively). This excellent performance is caused by the uniform dispersion of Fe3O4 nanoparticles on the graphene nanosheet surfaces, and thus achieving a large specific surface area and the structural integrity during the Liþ insertion/extraction process. © 2019 Elsevier B.V. All rights reserved.

Keywords: GNSs/Fe3O4 nanocomposites Anode material Solvent thermal method Lithium-ion batteries Electrochemical properties

1. Introduction Rechargeable lithium-ion batteries (LIBs), which have been considered as the most applicable energy-storage devices due to their high energy capacity and power density, are highly expected to be liberally applied in electric vehicles, hybrid electric vehicles and smart utility grids [1e6]. However, owing to the low theoretical capacity of commercial graphite anode (372 mAh g1), the current LIBs can hardly meet these requirements mentioned above and hinder the further application for LIBs. Accordingly, many efforts have been made to search for novel anode materials to obtain higher charge/discharge rate and reversible capacity as well as long cycle life and low cost. Transition metal oxides (TMOx, M ¼ Fe [7e12], Co [13e21], Ni [22e24], Mn [25e27], etc.) are recognized as one of the most likely candidates for promising anode materials due to the advantages of high theoretical capacities, high natural abundance, low cost, environmental benignity, etc. and their respectable performance as anode materials in LIBs. Among them, Fe3O4 has attracted close attention due to its high theoretical capacity (926 mAh g1), natural abundance, low cost and ecofriendliness [28e30]. Nevertheless, its dramatic volume variation and the agglomeration of active substance during the process of the

* Corresponding author. E-mail address: [email protected] (A. Zhu). https://doi.org/10.1016/j.jallcom.2019.152160 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Liþ insertion/extraction leads to the rapid capacity fading and poor rate performance, limiting its practical application. In addition, the low electrical conductivity of pristine Fe3O4 hinders the achievement of superior capacity at high charge/discharge rates [31e33]. In order to solve the problems mentioned above, two typical methods have been developed. One way is to construct nanostructured Fe3O4 materials with various morphologies, such as nanoparticles [34,35], nanorods [36,37], nanowires [38], nanocubes [39,40] and hollow spheres [41e43]. The other widely used way is to composite Fe3O4 with carbon material, including carbon-coated Fe3O4 nanostructures [44e46], graphene/Fe3O4 [8,47e50], and three dimensional (3D) graphene/[email protected] composite [51e53], etc. Among these various carbonous substrates, graphene is considered as the promising energy-storage material due to the high electrical conductivity, large surface area and light weight. For example, Feng and coworkers [54] reported a novel method to fabricate threedimensional graphene foams cross-linked with the Fe3O4 nanospheres which encapsulated with graphene and delivered a reversible specific capacity of 1059 mAh g1 at 93 mA g1 over 150 cycles. Wang et al. [55] synthesized a cornlike ordered N-doped carbon coated hollow Fe3O4 composites through a facile magnetic self-assembly method, which exhibited a capacity of 1316 mAh g1 at 100 mA g1 after 50 cycles. Huang et al. [56] reported a novel Fe3O4/rGO nanostructured composite which Fe3O4 nanorods are wrapped by the rGO nanosheets via covalent bonding and obtained a capacity of 1053 mAh g1 at 500 mA g1 after 250 cycles.

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Herein, we innovatively develop a novel method to synthesize graphene nanosheets loaded Fe3O4 nanoparticles. The GNSs/Fe3O4 NPs were fabricated via a freeze-drying graphene and iron ion suspension and followed by a solvent thermal synthesis method. The obtained Fe3O4 NPs can homogenously attach to the surface of graphene nanosheets. This composite microstructure morphology reduces the aggregation of Fe3O4 NPs greatly and increases the electrical conductivity of electrode material compared with Fe3O4 NPs. More importantly, the size of Fe3O4 NPs can be regulated from 20 nm to 100 nm by controlling the mass ratio of GNSs to Fe3þ of the suspension of graphene and Fe3þ, which has great influence on the electrochemical performance. A higher ratio of graphene to Fe3O4 NPs, the bigger of Fe3O4 particle size is. Our result demonstrates that the anode material of GNSs/Fe3O4 NPs with mass ratio of 1:2 exhibits excellent electrochemical performance: the fast charge-discharge capability at different current densities, an outstanding cycle performance during the long cycles and high reversible capacity (~1145 mAh g1 after 120 cycles).

2.3. Electrochemical characterization To measure the electrochemical performance of the prepared anode material, the standard CR2032-type coin cells were assembled in an argon-filled glovebox (H2O and O2 < 0.5 ppm). The asprepared GNSs/Fe3O4 NPs were directly performed as a working electrode without adding any other conducting agents. The electrolyte was 1 M LiPF6 solution in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, by volume). The coin-type cells (CR2032) were composed of the fabricated anode material, the electrolyte and lithium metal as counter electrode which were separated by Celgard 2325 polypropylene film. The galvanostatic charge-discharge (GCD) tests were performed at various current densities within a fixed voltage window of 0.01e3.0 V (vs. Liþ/Li) on Neware battery testing system (CT-4008, Neware, China). Cyclic voltammetry (CV, 0.1 mV s1, 0.01e3.0 V) measurements and the electrochemical impedance spectroscopy (EIS, 0.01 Hze100 kHz) tests were performed on CHI660 D electrochemical station (Chenhua, Shanghai).

2. Experimental section 3. Results and discussion 2.1. Materials and methods 2.1.1. Synthesis of GNSs/Fe3þ First, 130 mg graphene paste (purchased from Weihai Yunshan Technology Co., Ltd.) was added in 40 mL deionized water to obtain a homogenous suspension under ultrasonication treatment. Then, 1.35 g FeCl3$6H2O (AR, Shanghai Lingfeng Chemical Reagent Co., Ltd.) was added to the above graphene suspension with magnetic stirring for 0.5 h. Finally, freeze-drying the obtaining graphene and iron ion suspension, and Fe3þ uniformly loaded on graphene scaffold (GNSs/Fe3þ) were obtained.

2.1.2. Synthesis of GNSs/Fe3O4 nanoparticles (GNSs/Fe3O4 NPs) The prepared GNSs/Fe3þ was dispersed in 40 mL ethylene glycol (EG, AR, Sinopharm Chemical Reagent Co., Ltd.) under ultrasonication and stirring for 30 min. Then, 1 g PEG-4000 (AR, Sinopharm Chemical Reagent Co., Ltd.) and 3.6 g NaOAc (AR, Sinopharm Chemical Reagent Co., Ltd.) were added to the obtained suspension with further stirring for 1 h, and then was immediately transferred into a steel autoclave for solvothermal reaction for 18 h at 200  C. The GNSs/Fe3O4 NPs can be obtained by magnetic separation, washed with deionized water and ethanol for several times and drying in vacuum oven at 80  C for 12 h.

The fabricating process of GNSs/Fe3O4 NPs is illustrated in Scheme 1. As we can see from Scheme 1, Fe3O4 NPs homogenously attached to the surface of graphene nanosheets can be prepared via freeze drying of the suspension of graphene and Fe3þ and then solvent thermal synthesis method. The size of Fe3O4 NPs can be regulated from 20 nm to 100 nm by controlling the mass ratio of GNSs to Fe3þ of the suspension of graphene and Fe3þ, which may be caused by the space barrier effect. The X-ray diffraction was carried out to reveal the crystal structure and composition of the samples. The XRD pattern of the obtained GNSs/Fe3O4 with variable ratio of GNSs to Fe3O4 proportions is shown in Fig. 1a. The diffraction peaks of the composites at 30.3 (220), 35.5 (311), 43.1 (400), 57.3 (511), 62.8 (440) can be ascribed to the standard XRD data cubic phase Fe3O4 (JCPDS No.851436) and can be indexed to the cubic space group Fd3m. The average crystallite dimension of Fe3O4 nanoparticles was calculated by using Scherrer's formula, and the each estimated average crystal size of Fe3O4 nanoparticles is about 45.2 nm for GNSs/Fe3O4 NPs (1:1) (the ratio of GNSs to Fe3O4 of 1:1), 29.8 nm for GNSs/Fe3O4 NPs (1:2) and 20.9 nm for GNSs/Fe3O4 NPs (1:3). With Fe3O4 nanoparticle relative ratio increases, its size leads to decrease. Apart from these peaks, an additional peak located at the range of 25e28 is attributed to the (002) reflections of the stacked graphene nanosheets. Furthermore, the peak intensity of graphene is

2.2. Material characterization The microstructure and morphology of the samples were investigated by the Field-emission scanning electron microscope (FESEM S-4800II), transmission electron microscope (TEM Tecnai 12) and high-resolution TEM (HRTEM Tecnai G2F30). X-ray photoelectron spectroscopic (XPS) measurements were carried out using ESCALAB 250Xi system. The Brunauer-Emmett-Teller (BET) surface area was analyzed by nitrogen adsorptionedesorption isotherms at 77 K in Micromeritics ASAP 2020 system. The N2 adsorption isotherm was used to determine the pore size distribution via the Barret-Joyner-Halender (BJH) method. The average pore size was determined based on the nitrogen adsorption volume at a relative pressure of 0.993. Thermogravimetric analysis (TGA) (Pyris 1 TGA, PerkinElmer, USA) was performed under environmental atmosphere from room temperature to 800  C with a heating rate of 10  C min1.

Scheme 1. Illustration of fabricating process of GNSs/Fe3O4 NPs.

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Fig. 1. XRD patterns of GNSs/Fe3O4 NPs (1:1), GNSs/Fe3O4 NPs (1:2), GNSs/Fe3O4 NPs (1:3) and Fe3O4 NPs; TGA curves of GNSs/Fe3O4 NPs (1:1), GNSs/Fe3O4 NPs (1:2) and GNSs/ Fe3O4 NPs (1:3).

significantly affected by the mass ratio of graphene to Fe3O4 in the GNSs/Fe3O4 composite. The smaller the mass ratio, the weaker the peak intensity is. Also, no obvious extra diffraction peaks can be found in the XRD patterns, demonstrating that the prepared composite is relatively pure. Thermogravimetric analysis (TGA), tested under an air atmosphere, is used to calculate the proportion of each component of GNSs/Fe3O4 NPs, which is shown in Fig. 1b. The weight loss of carbon content is around 49.94 wt%, 33.35 wt% and 27.68 wt% for GNSs/Fe3O4 NPs (1:1), GNSs/Fe3O4 NPs (1:2) and GNSs/Fe3O4 NPs (1:3) respectively, which is very similar to the theoretical composition. To further confirm the chemical compositions of GNSs/Fe3O4 NPs, X-ray photoelectron spectroscopy (XPS) measurements were

performed. Fig. 2a shows the survey XPS spectrum of GNSs/Fe3O4 NPs (1:2), indicating the presence of carbon (C 1s), O (O 1s) and iron (Fe 2p) of the composite. The high-resolution XPS spectrum of Fe 2p (Fig. 2b) exhibited two main typical peaks (711.4 eV and 725.0 eV) of Fe3O4, which were corresponding to the Fe 2p3/2 and 2p1/2 states, respectively [57,58]. Moreover, this spectrum can be fitted with two spin-orbit doublets, accompanied by a shakeup satellite peak. The doubles are revealed to characteristic of the peaks of Fe2þ and Fe3þ [59,60]. Fig. 2c showed the results of O 1s spectrum, the high energy peak located at 530.5 eV corresponds to the anionic oxygen (O2) bonded with Fe3O4. In addition, the other two peaks located at 531.2 eV and 532.2 eV could be attributed to the chemisorbed oxygen caused by surface hydroxylation (such as OeH) [61,62]. For

Fig. 2. XPS spectra of the GNSs/Fe3O4 NPs: (a) survey spectrum, high-resolution XPS spectra of (b) Fe 2p, (c) O 1s and (d) C1s.

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the C 1s XPS spectrum (Fig. 2d), a strongest peak located at 284.8 eV, and the other two weak peeks at 285.8 eV and 287.1 eV can be assigned to CeC, C]O and OeC]O bonds respectively, which were associated with graphene carbon. XPS results confirm the successful synthesis of GNSs/Fe3O4 nanocomposites. The morphology and microstructure of the prepared GNSs/ Fe3O4 NPs are shown in Fig. 3. It can be clearly seen that the Fe3O4 NPs are homogenously attached to the surface of graphene nanosheets, showing good dispersion morphology. Also, the size of Fe3O4 nanoparticles is greatly affected by the ratio of GNSs to Fe3O4 NPs. Fig. 3g, h, i show the particle size distribution of Fe3O4 NPs on the surface of graphene nanosheets. It can be seen that the GNSs/ Fe3O4 NPs (1:1) has a particle size ranging from 50 to 130 nm (Fig. 3g) and the diameters for the GNSs/Fe3O4 NPs (1:2) are between 30 nm and 60 nm (Fig. 3h) and the GNSs/Fe3O4 NPs (1:3) has a particle size ranging from 16 nm to 26 nm (Fig. 3i). The average size of Fe3O4 nanoparticles on GNSs/Fe3O4 NPs (1:1) is 89 nm, which is larger than that of Fe3O4 nanoparticles (~40 nm) on GNSs/ Fe3O4 NPs (1:2) and (~21 nm) GNSs/Fe3O4 NPs (1:3). Fig. 4 shows the HRTEM image of GNSs/Fe3O4 NPs (1:2). From Fig. 4a, it can be seen that Fe3O4 nanoparticles shows 30e35 nm in diameter and Fig. 4b demonstrates the clear morphology of Fe3O4 nanoparticles loaded on the graphene nanosheets. As is shown in Fig. 4c and selected area electron diffraction pattern in Fig. 4d, the distance of the lattice fringes is approximate 0.258 nm, corresponding to the

(311) plane of Fe3O4. The specific surface areas and the porous structure of the samples are determined by measuring nitrogen adsorption-desorption isotherms at 77 K (Fig. 5). Remarkably, the Brunauer-Emmett-Teller specific surface area of GNSs/Fe3O4 NPs (1:2) is calculated to be 242.3 m2 g1 (Fig. 5a), which is much bigger than that of GNSs/ Fe3O4 NPs (1:1, 54.2 m2 g1) and GNSs/Fe3O4 NPs (1:3, 17.5 m2 g1). Based on the Barrett-Joyner-Halenda model, the pore size of the GNSs/Fe3O4 NPs samples ranges mostly between 2 and 11 nm, which confirms the existence of mesopores. The mesopores mainly originate from the aggregation of primary nanoparticles within Fe3O4 nanospheres and GNSs/Fe3O4 NPs as shown by the TEM results. This high surface area can be attributed to the composite microstructure morphology: small particle size of Fe3O4 NPs loaded on the surface of graphene nanosheets in the composite. Due to the high specific surface area and suitable distribution of pore size, the transport speed of Li ions can be largely improved and provide the extra active sites for the reaction of Li ions and accelerate mass diffusion of the electrolyte. Therefore, the GNSs/Fe3O4 NPs nanocomposite provides a good platform for lithium-storage property. The electrochemical properties of the GNSs/Fe3O4 NPs as Li-ion battery anodes were investigated using a two-electrode cell with lithium metal as the counter electrode. Fig. 6a shows the first four cyclic voltammograms (CVs) of GNSs/Fe3O4 NPs composite electrode cycled between 0.01 and 3.0 V (vs. Liþ/Li) at a scan rate of

Fig. 3. SEM and TEM images of GNSs/Fe3O4 NPs: (a) SEM of GNSs/Fe3O4 NPs (1:1) (b) TEM of GNSs/Fe3O4 NPs (1:1) (c) SEM of GNSs/Fe3O4 NPs (1:2) (d) TEM of GNSs/Fe3O4 NPs (1:2) (e) SEM of GNSs/Fe3O4 NPs (1:3) (f) TEM of GNSs/Fe3O4 NPs (1:3). (g, h, i) Particle size distribution of (a) Fe3O4 NPs (1:1), (c) Fe3O4 NPs (1:2) and (e) Fe3O4 NPs (1:3).

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Fig. 4. HRTEM images of (aed) GNSs/Fe3O4 NPs (1:2).

Fig. 5. (a) Nitrogen adsorption-desorption isotherms of GNSs/Fe3O4 NPs (1:1), GNSs/Fe3O4 NPs (1:2) and GNSs/Fe3O4 NPs (1:3); (b) the corresponding pore size distribution of GNSs/ Fe3O4 NPs (1:1), GNSs/Fe3O4 NPs (1:2) and GNSs/Fe3O4 NPs (1:3).

0.1 mV s1. Consistent with relevant literature, it is obvious that the CV curve of the first cycle is completely different from those of subsequent cycles, especially for the process of the lithiation. In the first cathodic scan, a strong reduction peak is observed at 0.44 V (vs. Liþ/Li), which is usually can be boiled down to two aspects. One is the reduction of Fe3þ or Fe2þ to Fe0 [Fe3O4 þ xLiþ þ xe / LixFe3O4], [LixFe3O4 þ (8-x)Liþ þ (8-x)e / 3Fe þ 4Li2O] and the other one is the irreversible reaction of side reactions on the electrode surfaces and interfaces due to the formation of SEI film [48,63e65]. During

the first anodic scan, the broad peak at about 1.67 V (vs. Liþ/Li) is associated with the reversible gradual oxidation of Fe0 to LixFe3O4 and Fe3þ, respectively. In the subsequent cycles, the cathodic and anodic peaks shift to 0.78 V (vs. Liþ/Li) and 1.90 V (vs. Liþ/Li), indicating the structural variations in the GNSs/Fe3O4 NPs electrode after Li-ion insertion in the first cycle. Meanwhile, it is noteworthy that the CV curves are highly overlapped after the first cycle, indicating a good electrochemical reversibility and stability of GNSs/Fe3O4 NPs electrode. Obviously, the peak intensity drops significantly in the second cycle, indicating the occurrence of some

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Fig. 6. (a) Cyclic voltammograms for the first four cycles of GNSs/Fe3O4 NPs; Charge-discharge voltage profiles of (b) GNSs/Fe3O4 NPs (1:1), (c) GNSs/Fe3O4 NPs (1:2), (d) GNSs/Fe3O4 NPs (1:3) at a current density of 0.1C (1C ¼ 1000 mA g1).

irreversible reactions with formation of SEI film. Apart from the redox reaction peak of Fe3O4, another pair of redox peaks located at 0.10 V and 0.27 V are also shown in the CV curve, corresponding to lithiation and delithiation of graphene nanosheets, respectively [66]. In the subsequent cycles, the distinct peaks appear at 0.78 V (vs. Liþ/Li) during discharge and at 1.8e1.9 V (vs. Liþ/Li) during charge, exclusively corresponding to the electrochemical reduction/oxidation (Fe3O44Fe) reactions accompanying lithium ion insertion (lithiation) and extraction (delithiation), in accordance with those previously reported in the literature for Fe3O4-based electrodes [67]. Apparently, the peak intensity drops significantly in the second cycle, indicating the occurrence of some irreversible reactions with formation of an SEI film. To further investigate the charge-discharge process of the GNSs/ Fe3O4 NPs anode material, Fig. 6 compares the 1st, 2nd, 3rd, 5th, 10th, 30th, 50th and 100th discharge/charge profiles of the GNSs/ Fe3O4 NPs (different mass ratios) at a current rate of 0.1C (1C ¼ 1000 mA g1) between 0.01 and 3.0 V (vs. Liþ/Li). The voltage plateaus can be observed at 0.78 V (vs. Liþ/Li), which correspond well to the results of the cyclic voltammetry test. Apparently, it can be seen that the composite electrode shows a higher capacity in the first cycle than the second cycle. Also, the capacity loss is probably owing to irreversible reactions by the formation of the SEI layer. In addition, it can be seen that the capacity of initial cycle decreases with the mass ratio of GNSs to Fe3O4 NPs (1835 mAh g1 for the mass ratio of 1:3, 1617 mAh g1 for the mass ratio of 1:2 and 987 mAh g1 for the mass ratio of 1:1). This result is relevant with the size of Fe3O4 nanoparticles, which the smaller size will result in the increment of specific surface area of electrode material and thus the

initial capacity. Compared to the GNSs/Fe3O4 NPs (1:1) and GNSs/ Fe3O4 NPs (1:2) electrodes, the GNSs/Fe3O4 NPs (1:3) electrode shows a fast specific capacity fading: 897.7 mAh g1 for the 5th cycle, 585.5 mAh g1 for the 30th cycle and 470.5 mAh g1 for the 50th cycle. To highlight the superiority of the unique GNSs/Fe3O4 NPs for anode materials of LIBs, we also tested the cycle performance of the GNSs/Fe3O4 NPs composite electrode at a constant current density of 0.1C (1C ¼ 1000 mA g1) between 0.01 and 3.0 V (vs. Liþ/Li). For comparison, the Fe3O4 nanoparticles were also investigated under the same synthetic method, and the results are shown in Fig. 7a. It can be seen that the specific capacity of the Fe3O4 nanoparticles drops rapidly from 1110 mAh g1 to 320 mAh g1 in the first 50 cycles, with the capacity retention rate of 40.9%. Apparently, the GNSs/Fe3O4 NPs nanocomposite electrode displays a higher reversible capacity of 1145 mAhg1 after 120 cycles and much better capacity retention (83.2%) compared with that of Fe3O4 NPs. After the Fe3O4 nanoparticles homogeneously attached to the surface of graphene nanosheets, it is worth noting that the reversible capacity of GNSs/Fe3O4 NPs electrode experiences a high capacity of 1046 mAh g1 during the initial 50 cycles, which is superior to the specific capacity of pure Fe3O4 NPs anode (about 320 mAh g1). The rate performance and corresponding coulombic efficiency of GNSs/Fe3O4 NPs (1:2) and commercial Fe3O4 NPs is compared in Fig. 7b. The average reversible capacities of GNSs/Fe3O4 NPs are 1179.1, 857.4, 750.8, 650.8, 529.7, 862.8 mAh g1 at the rates of 50, 100, 250, 500, 1000 and 100 mAh g1, respectively. The discharge capacity of GNSs/Fe3O4 NPs was reduced along with the increasing rate, as large currents can induce the polarization and reduce the diffusion process [68,69]. The capacity of GNSs/Fe3O4 NPs could be

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Fig. 7. Comparative cycling performance of different electrodes at a current density of 0.1 A g1 (a) in the first 120 cycles and (b) the cycle performance of GNSs/Fe3O4 NPs (1:2) over 0.01e3.0 V (versus Liþ/Li) at 1 A g1; (c) the rate capability and corresponding coulombic efficiency of GNSs/Fe3O4 NPs (1:2) and commercial Fe3O4 NPs at different current densities; (d) Nyquist plots of the Fe3O4 NPs and its nanocomposites in the frequency range from 100 kHz to 0.001 Hz; (e) equivalent circuit; (f) Z0 and u0.5 plots of GNSs/Fe3O4 NPs and Fe3O4 NPs.

recovered to 862.8 mAh g1 after reverted to 100 mA g1, which reveals a superior rate capability and reversible performance at high rates. Compared with the rate capabilities of commercial Fe3O4 NPs anode materials, the GNSs/Fe3O4 NPs anode is superior. In order to remark the excellent cycle stability of the GNSs/Fe3O4 NPs anode material at high rate at 1 A g1, Fig. 7c presents the (dis) charge capacities and corresponding coulombic efficiency of the GNSs/Fe3O4 NPs (1:2) electrode for 180 cycles. As can be seen, the first 10 cycles were performed at 0.1 A g1 and the 180 cycles at 1 A g1. Obviously, it can be seen clearly that the discharge capacity

of GNSs/Fe3O4 NPs anode material decays gradually in the first 45 cycles and then increases slowly and stabilizes to approximately 680 mAh g1 after 180 cycles, with a coulombic efficiency (CE) of almost 99% during the continuous lithiation/delithiation process. After 50 cycles, the GNSs/Fe3O4 NPs formed a more stable channel for the transportation of Li ions (as seen from the TEM image after cycling in Fig. S1). Moreover, the mesopores can effectively buffer the volume expansion during the cycling process. Also, the large specific surface area (from the BET test) can accelerate the transport of ions during the charge/discharge process. As a result, the

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Table 1 Performance comparison of LIB anode materials based on Fe3O4 structures (1C ¼ 1000 mA g1). Materials

Reversible capacity (cycle performance)/mAh g1

Rate capability/mAh g1

Voltage window/V (vs. Liþ/ Li)

Ref

Porous Fe3O4/carbon microspheres Fe3O4/hollow graphene spheres composite Hollow Fe3O4/graphene hybrid films Hollow Fe3O4/C/CNT microspheres Fe3O4 nanoparticles encapsulated in porous carbon fibers Graphene-wrapped Fe3O4 anode material Fe3O4 nanorods/N-doped graphene Single-crystalline mesoporous Fe3O4 nanorods GNS/Fe3O4 NPs

700 (300) @ 1C 1048 (50) @ 0.05C 940 (50) @ 0.2C 1200 (100) @ 0.2C 750 (300) @ 1C 680 (200) @ 1C

[email protected] [email protected] [email protected] [email protected] [email protected]

0.01e3.0 0.01e3.0 0.01e3.0 0.01e3.0 0.01e3.0

5 7 29 30 33

580 (10) @ 0.7C 929 (50) 0.1C 924 (50) @ 0.1C 1145 (120) @ 0.1C 650 (500) @ 1C

[email protected] [email protected] [email protected] [email protected]

0.01e3.0 0.01e3.0 0.05e3.0 0.01e3.0

35 36 32 this work

efficiency of the electric charge transfer can be improved, leading to the boost of capacity during the long cycles. This phenomenon has also been observed in the previous research for metal oxide electrodes, and the reasons of enhanced capacity can be summarized as the following points, which is well-documented in the literature [35,56,70,71]. First, along with the increase in the number of cycles, the electrode material is gradually pulverized, releasing more electroactive sites for Lithium ion storage. Meanwhile, the pulverized nanoparticles are still attached to the graphene nanosheets and hence the capacity gradually increases. Second, the 2D graphene nanosheets will increase the general conductivity of the electrodes (according to the results of A.C. impedance test). As a result, the efficiency of the electric charge transfer can be improved, leading to the boost of capacity during the long cycles. Third, the formation of polymeric/gel-like film on the surface of electrode materials can generate a so-called “pseudocapacitance-type” effect, which makes some contributions to enhance the reversible capacity [72,73]. The excellent reversible capacity of GNSs/Fe3O4 NPs anode material is mainly attributed to the nano-sized Fe3O4 particles loaded on the graphene nanosheets, and the resulting large specific surface area accelerates the transport of ions during the charge/discharge process. The lithium storage properties of GNSs/ Fe3O4 NPs anode is also found to be better than or comparable with typical Fe3O4-based anodes reported by other groups as shown in Table 1. For better understanding of the improved electrochemical performance of GNSs/Fe3O4 NPs compared with that of bare Fe3O4 NPs, EIS measurements were performed (Fig. 7d). All samples demonstrated one depressed semicircle at high frequency indicating charge transfer resistance (Rct) of the electrode/electrolyte interface as well as an inclined line at low frequency, showing Warburg impedance (Zw) related to the diffusion of lithium ions within the bulk of the electrode material. Also, the Liþ diffusion coefficient D can be calculated using Eq. (1):

RT D ¼ 0:5ð Þ2 AnF 2 sw C

(1)

where R is the gas constant, T is the absolute temperature, A is the area of the electrode surface, n is the number of electrons involved, F is Faraday's constant, C is the molar concentration of Liþ (mol cm3), and sw is the Warburg impedance coefficient which can be obtained from the slope of the line in Fig. 7f according to Eq. (2):

Z 0 ¼ Rs þ Rct þ sw u0:5

(2)

where u is the angular frequency in the low frequency region. Table 2 summarized the fitted charge transfer resistance, Warburg impedance and calculated Liþ diffusion coefficient of Fe3O4 NPs and

Table 2 The impedance parameters of Fe3O4 NPs and its nanocomposites.

GNSs/Fe3O4 NPs (1:1) GNSs/Fe3O4 NPs (1:2) GNSs/Fe3O4 NPs (1:3) Fe3O4 NPs

Rct/ohm

Ϭw/ohm cm2 s0.5

DLi/cm2s1

135.3 214.2 301.6 503.0

816.3 473.7 364.7 299.2

2.08  1016 6.18  1016 1.04  1015 1.55  1015

its nanocomposites. The total Rct of the GNSs/Fe3O4 composite based electrodes were determined to be 135.3 U (1:1), 214.2 U (1:2) and 301.6 U (1:3) which were significantly lower than that of Fe3O4 NPs based electrodes (503 U). With the gradual increase of the mass ratio of Fe3O4, the value of sw decreases gradually and the Fe3O4 NPs anode material has the least Warburg impedance and the largest Liþ diffusion coefficient, which means the Fe3O4 NPs anode material has the excellent rate performance. Comprehensively, these results indicate that the presence of graphene nanosheets can greatly improve their electrical conductivity, further proving the advantage of the GNSs/Fe3O4 NPs as LIB anode material. 4. Conclusions In summary, the novel graphene nanosheets loaded Fe3O4 nanoparticles have been successfully fabricated through a facile freeze-drying of graphene and iron ion suspension and followed by a solvent thermal synthesis strategy. In such a unique architecture, the diameter size of Fe3O4 NPs can be controlled in the range of 20 nme100 nm by controlling the mass ratio of graphene to Fe3O4 nanoparticles, and Fe3O4 nanoparticles demonstrate a uniform dispersion state. The graphene nanosheets can effectively alleviate the volume expansion as well as prevent the aggregation of Fe3O4 NPs and then maintain the structural integrity of GNSs/Fe3O4 NPs electrode during the Liþ insertion/extraction process. Moreover, the graphene nanosheets in GNSs/Fe3O4 nanocomposite provide the necessary conductive networks for electronic transmission without adding additional conductive agent. As a consequence, this electrode exhibits a high reversible capacity (~1145 mAh g1 after 120 cycles at 100 mA g1) and a remarkable rate capability (650 and 530 mAh g1 at 0.5 and 1 A g1, respectively). Acknowledgments This research was financially supported by a grant from the College and University Key Project of Jiangsu Province (No. 14KJA430006), Prospective United Innovation Project of Jiangsu Province (No. SBY2014020171) and Science and Technology Cooperation Funds of Yangzhou City and Yangzhou University (YZ2016250).

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152160.

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