Graphene composite as anode material for lithium-ion batteries

Graphene composite as anode material for lithium-ion batteries

Journal of Electroanalytical Chemistry 847 (2019) 113240 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal ho...

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Journal of Electroanalytical Chemistry 847 (2019) 113240

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Hierarchical sandwiched [email protected]/Graphene composite as anode material for lithium-ion batteries

T

Haipeng Lia, Jiayi Wanga, Yue Lia, Yan Zhaoa, , Yuan Tianb, Indira Kurmanbayevac, Zhumabay Bakenovc ⁎

a

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China Synergy Innovation Institute of GDUT, Heyuan, Guangdong Province, China c Institute of Batteries LLC, National Laboratory Astana, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan b

ARTICLE INFO

ABSTRACT

Keywords: [email protected]/Graphene composite Three-dimensional hierarchical Lithium ion battery Anode

A three-dimensional hierarchical sandwich structured [email protected]/Graphene nanocomposite was synthesized with a facile hydrothermal method, followed by carbonization process. The [email protected]/Graphene nanocomposite consists of well-dispersed carbon enwrapped Fe3O4 nanoparticles anchored on graphene layers. The three-dimensional sandwich structure and the wrapping carbon effectively improve the mechanical stability and conductivity of encapsulated Fe3O4. The as-prepared [email protected]/Graphene composite delivers a specific capacity of the 1253.3 mAh g−1 at the first cycle and 901.5 mAh g−1 after 200 cycles at 200 mA g−1. Even at 1500 mA g−1 , [email protected]/Graphene nanocomposite is able to deliver a discharge capacity of 592.3 mAh g−1. The good cycle and rate performance of [email protected]/Graphene anode can be attributed to its micro−/mesoporous structure with large specific surface area, which not only provides more transmission paths and active sites, but also reduces the diffusion impedance in electrolyte to realize fast transport of Li ions.

1. Introduction The rapid consumption of fossil fuel has led to a shortage of energy resources and dramatic ecological problems. These challenges can be addressed via the development of ecologically clean technologies. Therefore, zero-emission electric transport technologies are expected to dominate the automotive market in the near future. Nowadays, lithiumion batteries (LIBs) are widely used for electric equipment due to its high energy density. LIBs also are now the main energy source for portable electronics [1–3]. However, the extension of LIBs applications is still impeded by its limited energy capacity, toxicity of materials and high cost. Therefore, the development of new materials for LIBs is crucial. Advanced anode materials play a critical role in enhanced electrochemical and cost performance of LIBs. The industrialized anode for LIBs, graphite cannot meet the aforementioned challenges due to its poor capacity (372 mAh g−1), low initial charge-discharge efficiency, and undesirable ability to co-intercalate organic solvent into its structure, leading to its degradation [4]. To overcome these issues of the graphite anode, various high specific capacity and low-cost anode materials are developed. Among them, the metal oxides, such as SnO2, Mn3O4, ZnO, Fe2O3 and Fe3O4 etc. have attracted many attentions [4–7]. In particular, Fe3O4 is considered as a promising candidate due to its high theoretical ⁎

capacity (924 mAh g−1) and low cost. However, Fe3O4 suffers from a large volume variation (almost 180%) upon charge/discharge leading to its collapse, pulverization and loss of electric conductivity, negatively affecting its cycling performance and rate capability. There are two main strategies adopted to overcome these drawbacks of Fe3O4 anode. One of them is the fabrication of various micro- and nanostructures and/or architectures, including nanosphere [8], nanoflake [9], nanobox [10], nanoplate [11] etc. It has been reported that these nanostructures can not only accommodate the inherent large volume changes of Fe3O4, but also shorten the Li ions diffusion paths upon charge/discharge processes, remarkably enhancing kinetics and storing capability of Fe3O4. Another approach is the hybridization of Fe3O4 with electrically conductive material including carbon [12–14], metals and conducting polymers in order to enhance conductivity of the Fe3O4 anode and reinforce its structural stability [9,15]. In fact, carbon coating is considered as effective to accommodate the volumetric expansion of the Fe3O4 anode and improve its conductivity [16,17]. The carbon coated Fe3O4 nanomaterials have been successfully synthesized in our previous works and exhibited enhanced electrochemical performance [7,18]. However, their rate capability was limited due to a low dispersion. Graphene, as a two-dimensional (2D) material, is an excellent supporting material for nanocomposites for the LIBs

Corresponding author. E-mail address: [email protected] (Y. Zhao).

https://doi.org/10.1016/j.jelechem.2019.113240 Received 29 January 2019; Received in revised form 15 June 2019; Accepted 15 June 2019 Available online 17 June 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.

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applications, due to its large specific surface area (2630 m2 g−1), excellent electron mobility (2.5 × 105 cm2 V−1 s−1), and excellent mechanical flexibility [19,20]. Therefore, fabrication of graphene/Fe3O4 nanocomposites is very promising to meet the requirements for advanced anode materials. Recent studies have shown that the hybrid Fe3O4 with graphene can bring multiple synergistic effects. For example, the 2D structured graphene provides mechanical support for Fe3O4 nanoparticles, which can effectively alleviate the mechanical stresses and suppress intensive agglomeration of Fe3O4 nanoparticles [17]. Moreover, the Fe3O4 nanoparticles accumulate on the surface of graphene and effectively reduce stacking and agglomeration of graphene nanosheets [21]. Furthermore, graphene can greatly improve the electrical conductivity of the Fe3O4 system and reinforce its ionic transport abilities [22]. However, the synthesis of the Fe3O4/graphene composite with homogeneously dispersed Fe3O4 nanoparticles is challenging. In this work, we report a facile synthesis method for the development of well-dispersed [email protected]/Graphene nanocomposites with 3D hierarchical structure through hydrothermal reaction followed by carbonization process. The results indicate that the [email protected]/Graphene anode exhibits significantly improved cycling performance (901.5 mAh g−1 at 200 mA g−1 after 200 cycles) and rate capability (592.3 mAh g−1 at 1500 mA g−1) compared with a bare [email protected] anode.

2.2. Material characterization X-ray diffractometer (XRD, Rigaku D/Max 2500 V/pc) with Cu Kα radiation (λ = 0.15418 nm) was used to analyze the crystal structure, lattice parameters and crystallinity of the composites. Field emission scanning electron microscopy (SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2100F) were utilized to characterize morphology and microstructure of the composites. Thermogravimetric analysis (TGA, SDTQ-600) was applied to evaluate the content of each component in the composites, and the experiment was performed in the flowing air in the temperature range of 25–900 °C at a heating rate of 10 °C min−1. Raman spectra were acquired by a Raman spectrometer (DXR, Thermo Scientific) with an excitation wavelength of 532 nm. The Brunauer-Emmett-Teller (BET) surface area and non-local density functional theory (NLDFT) porous properties of the composites were calculated from the N2 adsorption-desorption isotherms measured at 77 K on an autosorb iQ instrument (Quantachrome Instruments). Fourier transformation infrared spectra (FTIR) were measured on the Bruker Vertex 80v spectrometer to analyze the surface composition of the composites. 2.3. Electrochemical property measurement Electrochemical performance was conducted in CR2025-type coin cells assembled in a glove-box filled with argon (MBRAUN, LAB master Glovebox, ≪0.1 ppm H2O and O2). The electrolyte is 1 M LiPF6 solution with a 1:1:1 mixture of ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (EC/DMC/EMC). Li foil and polypropylene film (Celgard-2300) was used as counter and as a separator, respectively. The working electrodes were prepared by mixing the active material ([email protected]/Graphene or [email protected]), conductive agent (acetylene black), and polymer binder (polyvinylidene fluoride, PVDF) with a weight ratio of 80:10:10 in N-methylpyrrolidinone (NMP, Sigma-Aldrich, ≥99.5% purity). Then, a homogenous slurry was spread on copper foil, keeping the density mass was about 2.2 mg cm−2. Charge-discharge tests were carried out using multichannel battery testers (CT-4008, Neware, and BT-2000, Arbin Inc.) between 0.05 and 3 V vs. Li/Li+ at various current densities. The electrochemical impedance spectroscopy (EIS) of cells with the composites electrodes were studied by electrochemical workstation (Im6e, Zahner) over a frequency range of 100 kHz to 0.1 Hz.

2. Experimental 2.1. Synthesis of [email protected]/Graphene and [email protected] Both of [email protected]/Graphene and [email protected] composites were synthesized via hydrothermal method followed by carbonization process [23]. Briefly, 0.9 g glucose (C6H12O6, ≥99%, Tianjin Fuchen Chemical Reagent Co.) and 1.212 g iron nitrate hydrate (Fe(NO3)3·9H2O, ≥98%, Tianjin Kewei Chemical Reagent Co.) were added into 40 mL deionized water and was stirred for 30 min. After that, 0.08 g graphene was added into the homogeneous solution, which was vigorously stirred for 30 min, and then put into an ultrasonic bath for 2 h at room temperature. The resulting mixture solution was poured into a Teflon®-lined stainless-steel autoclave vessel and hydrothermally treated at 190 °C for 9 h. When it cooled to room temperature, the obtained product was separated using a centrifuge, and washed with deionized water and anhydrous ethanol for several times. After dried overnight at 80 °C, the hydrothermal product was carbonized in a quartz tube furnace at 500 °C for 2 h in nitrogen with a heating rate of 5 °C min−1 to obtain the [email protected]/Graphene. For the purpose of comparison, [email protected] composite was obtained by the same synthesis method without graphene addition.

3. Results and discussion Fig. 1 presents the XRD patterns of graphene, [email protected]/Graphene and [email protected] Both [email protected]/Graphene and [email protected] show the characteristic diffraction peaks of cubic-structured Fe3O4 (JPDS No.

Fig. 1. (a) XRD patterns of graphene, [email protected]/Graphene and [email protected]; (b) TGA curves of [email protected]/Graphene and [email protected] 2

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Fig. 2. SEM images of (a, b) [email protected]/Graphene and (c, d) [email protected]; (e) TEM and (f) HRTEM images, (g) SEAD pattern of [email protected]/Graphene. Insets: (a) the morphology of [email protected]/Graphene after 20 cycles, (b, d) the particle size distribution of [email protected] in samples.

65–3107). There are no other obvious diffraction peaks in the XRD patterns of [email protected]/Graphene, implying other oxide impurity exits in the sample. Interestingly, the diffraction peaks of graphene at 2θ of 23.4° and 43.2°, were not observed in [email protected]/Graphene, which can be explained by the lesser stack and accumulation of graphene sheets due to attaching [email protected] on the surface of graphene, or by a more arrangement disorder and less content of graphene in the composite [20,24]. The average crystalline sizes of the samples were calculated on the basis of XRD results using the Debye-Scherrer's formula:

D=

36.08 wt%, respectively, i.e. the proportion of graphene in [email protected]/ Graphene was about 11.2 wt%. Fig. 2a and b show the SEM images of [email protected]/Graphene nanocomposite, it can be seen that the spherical [email protected] nanoparticles are uniformly attached to graphene surfaces which the average size is about 51.2 nm (Inset: Fig. 2b). Compared with the [email protected] (Fig. 2c, d), the [email protected]/Graphene nanocomposite has similar particle sizes but narrower particle size distribution (inset Fig. 2d) and better dispersion. It can be seen that intertangling and restacking of graphene layers and the aggregation of [email protected] nanoparticles during the high pressure hydrothermal process were effectively suppressed [25]. After annealing, the [email protected]/Graphene nanocomposite with uniformly dispersed particles was obtained. The morphology of [email protected]/Graphene after 20 cycles was performed in Fig. 2a, which illustrates complete morphology was still maintained after battery test. Excellent structural stability of [email protected]/Graphene was proven, meaning it can be used as a candidate for anode materials in lithium ion battery. The structure of [email protected]/Graphene composite was further investigated by TEM (Fig. 2e–g). As shown in Fig. 2e, the [email protected] nanoparticles are pomegranate-shaped and are composed of multiple Fe3O4 nanocrystals wrapped by a carbon layer. Furthermore, the

0.89 . cos

In the formula, D is the Fe3O4 crystallites size, λ is the X-ray wavelength, β is the peak FWHM (full width at half maximum) in radian, and θ is the Bragg's diffraction angle. The average Fe3O4 crystalline size of [email protected]/Graphene is 11.7 nm, which is identical to the result for [email protected] (10.8 nm). Fig. 1b presents the TGA results of [email protected]/ Graphene and [email protected] in air. The TGA curves of the two samples show similar trends of weight loss, including oxidation of Fe3O4 to Fe2O3 and oxidation of C of the composites to CO2. The amount of carbon in [email protected] and [email protected]/Graphene were determined as 24.86 wt% and 3

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Fig. 3. Raman spectra (a) and FT-IR spectra (b) of graphene, [email protected]/Graphene and [email protected]

[email protected] nanoparticles are uniformly anchored on the graphene layers and form a 3D hierarchical sandwich structure. It is worth to mention that, even after a long time ultrasonic treatment during sample preparation for TEM, the [email protected] nanoparticles are still well-distributed and adhere to graphene, suggesting a strong interaction between [email protected] and graphene [26]. Fig. 2f shows the high-magnification TEM image of [email protected]/Graphene. It demonstrates that the 10–12 nm Fe3O4 nanocrystals have an interplanar spacing of 0.256 nm and 0.203 nm, which corresponding to the lattice planes (311) and (400) of a cubicstructured Fe3O4. The carbon layer is amorphous with a thickness of about 4 nm. The SEAD pattern (Fig. 2g) shows well-defined multiple diffraction rings also corresponding to a cubic-structured Fe3O4, implying the polycrystalline properties of [email protected]/Graphene nanocomposite. Fig. 3a shows the Raman spectra of graphene, [email protected]/Graphene and [email protected] between 400 and 3500 cm−1. There are two peaks appearing at ~1370 cm−1 and ~1590 cm−1 of all the samples corresponding to the D-band and G-band peaks of graphite. The D-band is the breathing mode with A1g symmetry of disordered carbon atoms, whereas G-band is related to the vibration of 2D six-party lattice of carbon atoms. The intensity ratio of D-band and G-band (ID/IG) can be utilized to assess the crystallinity degree for carbon materials. The ID/IG of [email protected]/Graphene and [email protected] are calculated to be 0.72 and 0.79, respectively, indicating the higher graphitization degree and improved conductivity of [email protected]/Graphene [27]. In addition, the ID/IG value (0.69) of pure graphene is lower than [email protected]/Graphene, which might be resulted from the damage of the graphene lattice structure during hydrothermal synthetic process. Fig. 3b shows the FTIR spectra of graphene, [email protected]/Graphene and [email protected] The absorption bands observed at 876 cm−1, 1045 cm−1, 1642 cm−1, 2922 cm−1 and 3413 cm−1 in the spectrum of graphene can be assigned to the stretching vibrations of ]CeH, CeO, C]C, CeH and OeH [28]. The FTIR spectra of [email protected]/Graphene and [email protected] exhibit absorption peaks at 579 cm−1, which belong to the FeeO stretching vibration [29]. The high intense ]CeH and CeO stretching vibrations of graphene at 876 cm−1 and 1045 cm−1 completely disappear in [email protected]/ Graphene, whereas the C]C stretching vibration at 1642 cm−1 was strengthen significantly. The FTIR results demonstrate that [email protected] was combined with graphene and formed stable chemical bond [30]. To further study the porous nature and surface area of [email protected]/ Graphene and [email protected], their N2 adsorption-desorption isotherms were obtained. In Fig. 4a and c, N2 adsorption-desorption isotherms of [email protected] and [email protected]/Graphene present a type-I and type-IV isotherm plots with typical H3 hysteresis loop, respectively, indicating micro- and meso-porosity of the two composites [31]. The pore size distribution (PSD) curves of the two samples (Fig. 4b and d) were obtained from the N2 isotherms by Non Localized Density Functional Theory (NLDFT) method. As shown in Fig. 4d, the PSD curve displays

concentrated existence at 0.53 nm, 1.41 nm, 2.77 nm and 4.11 nm, confirming hierarchical porosity of [email protected]/Graphene. The key pore characteristics including BET surface area (SBET), micropore surface area (Smicro), mesopore surface area (Smeso) and total pore volume (Vtot) for the [email protected]/Graphene and [email protected] are summarized in Table 1. SBET of [email protected]/Graphene is 219 m2 g−1, which is increased by nearly 11% as compared with that of [email protected] (197 m2 g−1). Also, Smicro for [email protected]/Graphene (124 m2 g−1) is larger than that of [email protected] (109 m2 g−1). However, Vtot of [email protected]/Graphene (0.285 cm3 g−1) shows significant reduction compared with [email protected] (0.491 cm3 g−1). The larger Smicro in [email protected]/Graphene can be explained by the lattice graphene imperfections and massive micropores caused by the amorphous carbon layer of [email protected] during synthesis. On the other hand, the uniformly dispersed [email protected] on the graphene sheets can effectively reduce the graphene agglomeration and conglutination, resulting in the Vtot decrease for [email protected]/Graphene. A large specific surface area can reduce the diffusion impedance in the electrolyte and realize fast transport of Li ions and electrons during charge/discharge processes [31]. Meanwhile, micro- and meso- porous structure can effective impede the volume expansion/contraction of Fe3O4 nanocrystals, which can improve the cycling stability of the electrodes [32]. Based on the above XRD, SEM, TEM and FTIR results, a formation mechanism of [email protected]/Graphene is proposed (Fig. 5). According to the FTIR results, the graphene surface contains abundant C]O and CeH bands, which is beneficial for formation of stable chemical bonding between graphene and [email protected] [33,34]. During the ultrasonic processing, graphene is dispersed into reaction solution, which fills the interval and pleats of graphene. As the temperature and pressure increases during hydrothermal reaction, the C6H12O6 experience dehydration, polycondensation and aromatization, and then combined with –C]O and –CeH groups of graphene and forms carbonaceous colloids on graphene. Meanwhile, Fe(NO3)3 would hydrolyze with H2O molecules and forms Fe(OH)3 gel, which is further dehydrated and reduced under high temperature and pressure, and subsequently forming the Fe2O3 nanocrystals. Finally, the Fe2O3 nanocrystals are combined with carbonaceous colloid through the coulomb forces to produce [email protected] on graphene surface [23,35]. Upon the high-temperature processing, the Fe2O3 nanocrystals are reduced by carbon to Fe3O4 nanocrystals anchored on grapheme sheet. The electrochemical performance of the [email protected]/Graphene and [email protected] composites were investigated by charge-discharge cycling at 200 mA g−1 between 0.05 and 3 V. Fig. 6a–c display the charge-discharge profiles and dQ/dV vs. potential curves of the [email protected]/Graphene electrode in the 1st, 2nd, 10th, 50th, 80th and 120th cycles. Discharge capacities of [email protected]/Graphene at the corresponding cycles are 1253.3 mAh g−1, 1108.0 mAh g−1, 825.3 mAh g−1, −1 −1 −1 805.1 mAh g , 831.6 mAh g and 856.2 mAh g , respectively. It is noteworthy that the discharge capacity of [email protected]/Graphene 4

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Fig. 4. N2 adsorption-desorption isotherms of (a) [email protected] and (b) [email protected]/Graphene. PSD curves of (c) [email protected] and (d) [email protected]/Graphene.

of graphene [28]. In the first charge curve, there is a slope potential rise between 1.5–2.0 V, which corresponds to a multi-step oxidation of Fe0 [26]. In the following discharge curves, the short potential plateau disappears. At the same time, the long plateau shifts towards the higher potentials due to the formation of irreversible solid electrolyte interface (SEI) and change in polarization of the electrode during the initial cycling process [38]. Fig. 6b and c show that the electrochemical polarization of [email protected]/Graphene decreases as during the charge/discharge cycling, which leads to the increase of discharge potential plateau [39]. Three peaks in the discharge (Fig. 6(b)) located at 0.73, 0.85, 0.91 V were considered to be the formation of SEI and reaction of Fe3O4 with Li. The dQ/dV vs. voltage curves show obvious similarities (Fig. 6(c)) in different cycles, which implies the reversibility of the redox reaction. Fig. 6d presents the cycling performance of [email protected]/Graphene and [email protected] at 200 mA g−1, and their initial discharge capacities are 1253.3 mAh g−1 and 1123.6 mAh g−1, respectively. However, the initial coulombic efficiency of [email protected] (78.7%) is higher than that of [email protected]/Graphene (70.1%), which might be resulted from the different pore structures of two materials. Due to a larger specific surface area of [email protected]/Graphene (219 m2 g−1), there is a larger number of Li ions embedded into micro- and mesopores of [email protected]/Graphene than that of [email protected] during formation of SEI film. This would partly exhaust more Li ions and resulting in a lower coulombic efficiency in [email protected]/Graphene [40]. After 100 cycles, the reversible capacities of [email protected]/Graphene and [email protected] are 845.3 mAh g−1 and

Table 1 BET results (surface area and pore volume) for the [email protected]/Graphene and [email protected] Samples

SBET (m2 g−1)a

Smicro (m2 g−1)b

Smeso (m2 g−1)c

Vtot (cm3 g−1)d

[email protected] [email protected]/ Graphene

197 219

109 124

88 95

0.491 0.285

a SBET = specific surface area calculated by using Brunauer-Emmett-Teller (BET) method. b Smicro = micropore surface area calculated by t-plot method. c Smeso = mesopore surface area obtained by Smicro subtract from SBET. d Vtot = total pore volume calculated by NLDFT method.

increases slightly from 50th cycle and then becomes stable. The capacity enhancement can be attributed to reversible formation of polymer electrolyte membrane through degradation of the electrolyte, which was also reported in other metal oxide composite materials [36,37]. A short potential plateau can be observed at around 0.95 V in the first discharge cycle, which is associated with an initial irreversible step: Fe3O4 + xLi++xe− → LixFe3O4. Subsequently, Fe and Li2O were transformed: LixFe3O4 + (8-x)Li++(8-x)e−↔Li2O + 3Fe [14–18]. After Fe3O4 fully reacted with Li+, a lithiation reaction of graphene between 0.5 and 0.05 V at the discharge curve occured: 2C + Li+ + e− → LiC2, suggesting a portion of capacity contribution 5

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Fig. 5. The fabrication process of [email protected]/Graphene.

741.5 mAh g−1, respectively. Moreover, [email protected]/Graphene still remains a stable reversible capacity of 901.5 mAh g−1 with a low capacity decay of only 0.1% per cycle even after 200 cycles. Fig. 6e shows the rate performances of the samples investigated at different current rates from 200 mA g−1 to 1500 mA g−1. The results show that [email protected]/Graphene exhibits specific capacities of 931.5 mAh g−1, 779.3 mAh g−1, 693.2 mAh g−1, 592.3 mAh g−1, and 854.9 mAh g−1 at current densities of 200 mA g−1, 500 mA g−1, 1000 mA g−1, 1500 mA g−1 and again at 200 mA g−1, respectively. [email protected]/Graphene exhibits better capacity retention of 592.3 mAh g−1 at 1500 mA g−1 than [email protected] (512 mAh g−1). Furthermore, when the current rate returns to 200 mA g−1, the capacity of [email protected]/Graphene is recovered to 854.9 mAh g−1 indicating rapid reaction kinetics of Li ions insertion/extraction in [email protected]/Graphene. In order to investigate the effect of conductivity of graphene addition on composite, the electrochemical impedance spectroscopy (EIS) of [email protected]/Graphene and [email protected] were carried out after three chargedischarge cycles. The Nyquist plots for both materials (Fig. 6f) are composed of semicircles in the high frequency region, which is attributed to charge transfer resistance (Rct) and the electrolyte/electrode interface resistance (Rf). The semicircle is followed by an inclined line in the low frequency zone, which corresponding to the Warburg diffusion (Rzw) responsible for diffusion resistance of Li ions in the electrode. The intercept of the impedance spectra with the imaginary axis at high frequency represents the electrolyte resistance (Re) [41]. The corresponding equivalent circuit fitting (inset of Fig. 6f) was obtained by analyzing Nyquist plots using Zsimpwin software (Princeton Applied Research Inc.), where Cf is the double layer capacitance of interface of electrolyte/electrode and Cct corresponds to the diffusion capacitance.

The values of Re, Rf and Rct for two electrodes are presented in Table 2. It shows that the Rf and Rct of [email protected]/Graphene electrode are smaller than that for [email protected] The reduced charge transfer and electrolyte/ electrode interface resistance confirm the conductivity enhancement due to the addition of graphene in [email protected]/Graphene. The electrochemical enhancement of [email protected]/Graphene electrodes can be attributed to the following three factors: i) [email protected]/ Graphene possesses a more stable 3D hierarchical sandwich structure. The pomegranate-shaped [email protected] nanoparticles are tightly anchored on graphene and can not only suppress the restacking of graphene layers, but also impede the aggregation of [email protected] nanoparticles during the Li ions insertion/desertion; ii) Due to the homogenous dispersion of [email protected] nanoparticles, the carbon shell was combined with graphene through hydrothermal reaction to form 3D hierarchical conductive network, which can effectively improve the rapid conduction of electrons and ions and reduce the internal resistance of electrode material; iii) [email protected]/Graphene have micro-mesoporous structure, and provide more transmission paths and active sites. Meanwhile, the diffusion impedance in electrolyte is lowered to realize fast transport of Li ions. 4. Conclusions In summary, three-dimensional layered [email protected]/Graphene nanocomposites with a sandwich structure were fabricated by a simple hydrothermal method and followed by carbonization. In [email protected]/ Graphene, [email protected] nanoparticles (~51.2 nm) were uniformly distributed on the graphene layer forming a 3D layered interlayer structure. When applied as anode material for lithium-ion battery, 6

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Fig. 6. (a) Charge-discharge profiles and (b, c) dQ/dV vs. voltage curves of [email protected]/Graphene between 0.05 and 3 V at a current density of 200 mA g−1; (d) Cycling performance (current density of 200 mA g−1), (e) Rate capabilities and (f) Nyquist profiles of the [email protected]/Graphene and [email protected] anodes after 3 cycles. Inset: the corresponding equivalent circuit.

stability and rate capability. Therefore, as compared with the [email protected] composite, the [email protected]/Graphene composite possess remarkably enhanced electrochemical performance, showing that it can be used as a candidate for anode materials for lithium ion battery.

Table 2 Impedance data of [email protected]/Graphene anode and [email protected] anode. Samples

Re (Ohm)

Rf (Ohm)

Rct (Ohm)

[email protected] [email protected]/Graphene

2.92 3.93

22.79 17.02

0.87 0.61

Declaration of Competing Interest There are no conflicts to declare.

[email protected]/Graphene delivers a high reversible capacity of 901.5 mAh g−1 even after 200 cycles at 200 mA g−1. It also delivers an exceptionally high specific capacity of ≫592.3 mAh g−1 at a high current density of 1500 mA g−1. The 3D hierarchical sandwich structured [email protected]/Graphene nanocomposite could effectively improve the mechanical stability and conductivity of Fe3O4, leading to great cycle

Acknowledgements This work was supported by the financial support from the Program for the Outstanding Young Talents of Hebei Province; Natural Science Foundation of Hebei Province of China [grant number E2015202037]; 7

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H. Li, et al.

Science and Technology Correspondent Project of Tianjin [grant number 14JCTPJC00496]; Research project [grant number AP05133706] by the Ministry of Education and Science of Kazakhstan; Cultivation project of National Engineering Technology Center [Grant No. 2017B090903008].

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