Self-assembly of hierarchical Fe₃O₄ microsphere/graphene nanosheet composite: towards a promising high-performance anode for Li-ion batteries

Abstract

A hierarchical Fe₃O₄ microsphere/graphene nanosheet (H-Fe₃O₄-MS/GNS) composite has been synthesized by a facile one-pot solvothermal route. The Fe₃O₄ microspheres uniformly decorated the surface of the two dimensional GNS. Each Fe₃O₄ microsphere possesses a hierarchical and porous structure, which is composed of Fe₃O₄ nanoparticles with a diameter of about 10 nm. As an anode material for Li-ion batteries, the H-Fe₃O₄-MS/GNS composite shows high specific capacity and good cycling stability (1171.6 mA h g⁻¹ at 200 mA g⁻¹ and 940.4 mA h g⁻¹ at 500 mA g⁻¹ up to 70 cycles), reduced voltage hysteresis, as well as enhanced rate capability. The improved electrochemical performance can be attributed to the combination of the conductivity, confinement and dispersion effects of GNS and the porous hierarchical structure of the Fe₃O₄ microsphere.

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Introduction

With the increasing demand of Li-ion batteries (LIBs) for promising power sources in electric vehicles (EV) and hybrid electric vehicles (HEV), much attention has been paid to alternative anode materials with high capacity, long cycling lifetime and high safety.^1–6 So far, transition metal oxides such as Co₃O₄,⁷ TiO₂,⁸ FeO_x,^4,9 MnO¹⁰ have been intensively investigated as anode materials for LIBs due to their high specific capacities. Among them, Fe₃O₄ has received particular attention due to its high specific capacity, low cost, natural abundance and environmental friendliness. However, there still exist critical problems that limit its practical applications, such as intrinsically low electrical conductivity, large volume expansion/contraction during Li-ion insertion/extraction and large voltage hysteresis between the discharge and charge steps, consequently, result in poor cycling stability.^11,12

Great effort has been made to improve the electrochemical performance of Fe₃O₄ including construction nano-sized Fe₃O₄ with different morphologies or architectures,^4,13,14 since it is an effective strategy to improve the cyclability due to its large surface area which would result in the sufficient contact of active material/electrolyte and the short diffusion length of Li ions. Another strategy is combining Fe₃O₄ with carbonaceous materials,^15–25 because these carbonaceous materials not only increase the electrical conductivity but also suppress the particle aggregation, as well as the buffering effect for large volume changes. Nevertheless, achieving a long cycle life and a good rate capability still remains a large challenge for these materials.

Graphene nanosheet (GNS), a two-dimensional one-atom-thick sheet of sp²-bond carbon atoms,²⁶ has been reported with extraordinarily high electrical conductivity, unique mechanical properties, and an ultrahigh specific surface area.^27,28 Recently, graphene-based transition metal oxides have been widely fabricated for rechargeable LIBs, such as Mn₃O₄–GNS,²⁹ Co₃O₄–GNS,^30,31 Fe₃O₄–GNS,^11,32–35 Fe₂O₃–GNS,^36,37 SnO₂–GNS,³⁸ TiO₂–GNS,^39,40 NiO–GNS.⁴¹ These composite anodes have demonstrated large reversible capacities, high cycling stability and performances, attributing to the GNS which can (a) assist to accommodate volume expansion/contraction and avoid aggregation of active materials, (b) act as structural buffer during charge–discharge process, (c) improve the electrical conductivity of anode materials and (d) accelerate Li ion and electron transport by providing a shortened diffusion pathway.

Herein, we prepare a hierarchical Fe₃O₄ microsphere/graphene nanosheet (H-Fe₃O₄-MS/GNS) composite by a facile one-pot solvothermal route. In fact, hierarchical Fe₃O₄ sphere/graphene composites with different morphologies have perfect properties, such as high-performance microwave absorption ability,^42,43 excellent water dispersibility,⁴⁴ high magnetism^45–47 and so on. As anode materials for LIBs, they present excellent electrochemical performance. Li et al.⁴⁸ reported a three-dimensional (3D) hierarchical Fe₃O₄/graphene nanosheet (GNS) composite, of which the Fe₃O₄ nanoflowers were highly encapsulated in a GNS matrix. Jin et al.⁴⁹ prepared the Fe₃O₄–pyrolytic graphite oxide (Fe₃O₄–PGO) composite of PGO sheets decorated with microsphere-type Fe₃O₄. Chen et al.⁵⁰ reported the synthesis of a novel hollow porous Fe₃O₄ bead-rGO composite structure. In our synthetic approach, ethylene glycol (EG) was used as both a reducing agent and solvent. Sodium acetate was used not only for electrostatic stabilization to prevent the agglomeration of the particles but also to assist in the reduction of Fe³⁺ to Fe₃O₄. The negatively charged graphene oxide (GO) sheets could absorb positively charged Fe³⁺ by the electrostatic attraction in the oxygen-containing groups, which might serve as the nucleation sites.¹² Even a more important is that additional sonication can help the Fe³⁺ to be adsorbed onto the surface of the GO powerfully. Compared with their composites, the hierarchical Fe₃O₄ microspheres we synthesized possess smaller secondary particles with a diameter of about 10 nm, which means that a shorter Li ion diffusion pathway. This composite shows a higher specific capacity and a better cycling stability, as well as enhanced rate capability. The excellent electrochemical performance can be attributed to a combination of GNS with hierarchical Fe₃O₄ microspheres.

Experimental

Synthesis of H-Fe₃O₄-MS/GNS composite

GO was prepared from natural graphite by the modified Hummers' method.⁵¹ H-Fe₃O₄-MS/GNS composite was fabricated by simultaneously forming Fe₃O₄ microspheres and reducing GO in EG. In a typical process, 50 mg of GO was added to 50 mL of EG with sonication for 3 h to form a uniform dispersion. Then, 1.5 mmol of FeCl₃·6H₂O were dissolved in EG (10 mL) and then added slowly to the above dispersion under stirring and then sonicate again to make the Fe³⁺ to be adsorbed onto the surface of the GO powerfully, followed by the dropwise addition of 10 mL of an EG solution mixed with sodium acetate (0.5 g) and stirring several hours. The mixed dispersion was then transferred to a Teflon-lined stainless steel autoclave and heated in an electric oven at 180 °C for 24 h. After cooling down to room temperature, the product was centrifuged and washed several times by alcohol and deionized water before drying at 80 °C in an oven overnight. The resulting powder was loaded into a tube furnace and heated under a argon gas atmosphere from room temperature to 500 °C at a heating rate of 10 °C min⁻¹, maintaining at this temperature for 3 h to obtain a well-crystalline H-Fe₃O₄-MS/GNS composite. The formation mechanism is schematically illustrated in Fig. 1. For comparison, crystalline Fe₃O₄ microspheres were also obtained using the same route without addition of GO.

Fig. 1 Schematic illustration of the formation mechanism of H-Fe₃O₄-MS/GNS composite.

Structural characterization

The morphology and microstructure of the products were characterized by X-ray diffraction (XRD, Rigaku D/max 2550 PC, Cu Kα), X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD), scanning electron microscopy (SEM, Hitachi S-4700 and FESEM, FEI Sirion-100), and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). The Raman spectra were measured on a Jobin Yvon Labor Raman HR-800 using Ar-ion laser of 514.5 nm. Thermogravimetric (TG) analysis was conducted on a SDT Q600 instrument from 30 to 800 °C at a heating rate of 5 °C min⁻¹ in air.

Electrochemical measurements

The electrochemical tests were performed using a coin-type half cell (CR 2025). The working electrodes were prepared by a slurry coating procedure. The slurry consisting of 95 wt% H-Fe₃O₄-MS/GNS composite and 5 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP) was pasted on copper foil. After dried at 90 °C for 24 h in vacuum, the samples were pressed under a pressure of 20 MPa. Test cells were assembled in an argon-filled glove box with the metallic lithium foil as the as both the reference and counter electrodes, 1 M LiPF₆ in ethylene carbonate (EC)–dimethyl carbonate (DME) (1 : 1 in volume) as the electrolyte, and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator.

The galvanostatic charge–discharge tests were conducted on a LAND battery program-control test system at certain current densities between 0.01 and 3.0 V at room temperature (25 ± 1 °C). Cyclic voltammetry (CV) tests were performed on the CHI660C electrochemical workstation in the potential range of 0–3.0 V (vs. Li/Li⁺) at a scanning rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 100 kHz to 0.01 Hz under AC stimulus with 5 mV of amplitude.

Results and discussion

The content of GNS in H-Fe₃O₄-MS/GNS composite was determined based on TG analysis (see the ESI, Fig. S1†). From the TG plot, small weight loss of about 2% below 150 °C is ascribed to the evaporation of moisture in the composite. A weight increase of 0.26% is observed because of the oxygenation of Fe₃O₄. With increasing the temperature, the combustion of the GNS began at about 200 °C and completed at about 500 °C. It can be estimated that the weight percentage of the GNS in the composite is 23%.

Fig. 2 shows typical XRD patterns of the H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄. The diffraction peaks of the two powders are assigned to the pure face-centered cubic structural (Fd3m space group) magnetite Fe₃O₄ (JCPDS card no. 19-0629). The H-Fe₃O₄-MS/GNS composite shows an additional broad (002) diffraction peak at about 25°, which can be indexed into the disorderly stacked and less agglomerated GNS in the composite.⁵²

Fig. 2 XRD patterns of (a) H-Fe₃O₄-MS/GNS composite, and (b) bare Fe₃O₄.

The Raman spectra of H-Fe₃O₄-MS/GNS composite and GO provide the information related to the carbon structural changes during the synthesis process (Fig. S2†). Two prominent peaks at 1357 cm⁻¹ and 1592 cm⁻¹ are observed in GO, corresponding to the well-documented D and G bands, respectively. The D band is associated with disorder carbon, while the G band corresponds to sp² hybridized carbon.⁵³ A general feature of the reduction of GO is that the G band shifts to a lower wavenumber. The Raman spectrum of H-Fe₃O₄-MS/GNS composite also possesses both of the D and G bands, but the G band shifts to 1589 cm⁻¹, indicating the reduction of GO. Another Raman spectrum feature of the reduction of GO is that the ratio of the two bands, D/G, increases when GO is reduced. The D/G intensity ratios are 0.89 for GO and 1.12 for H-Fe₃O₄-MS/GNS composite. This change in D/G intensity ratio indicates the presence of localized sp³ defects within the sp² carbon network upon reduction of the exfoliated GO.⁵⁴

The surface compositions of GO and H-Fe₃O₄-MS/GNS composite are further confirmed by XPS measurements. The XPS spectrum is fitted by the method of Gaussian fitting. As shown in Fig. 3a, the C 1s XPS spectrum of GO contains five components of (i) the nonoxygenated C at 284.6 eV, (ii) the carbon in C–OH at 285.7 eV, (iii) the carbon in C–O at 286.7 eV, (iv) the carbonyl carbon (C=O) at 288.0 eV and (vi) the carboxylate carbon (O=C–O) at 289.1 eV. As shown in Fig. 3b, however, the C 1s XPS spectrum of H-Fe₃O₄-MS/GNS composite only includes three components. Furthermore, the relative contribution of the components associated with oxygenated functional groups decreases markedly, indicative of a sufficient reduction of GO. The inset of Fig. 3b gives the XPS survey spectrum of the H-Fe₃O₄-MS/GNS composite in the region of 0–1200 eV. The detected elements are Fe, C and O as expected for the H-Fe₃O₄-MS/GNS composite. The Fe 2p and O 1s XPS spectra confirm the presence of Fe₃O₄.

Fig. 3 C 1s XPS spectra of (a) GO, and (b) H-Fe₃O₄-MS/GNS composite (XPS survey spectrum of the H-Fe₃O₄-MS/GNS composite presented in inset).

Fig. 4a clearly shows that the Fe₃O₄ microspheres uniformly decorated on the surface of the two-dimensional GNS. Each Fe₃O₄ microsphere has a diameter of about 200 nm. In contrast, the bare Fe₃O₄ microspheres tend to aggregate without the immobilizing effect of GNS and have a wide size distribution (Fig. 4b). TEM images of H-Fe₃O₄-MS/GNS composite confirm the SEM results and reveal that the Fe₃O₄ microspheres possess a hierarchical and porous structure. The translucent GNS is homogeneously covered by Fe₃O₄ microspheres (Fig. 5a and b). The GNS shows the folding nature which is clearly visible. A single Fe₃O₄ microsphere which sticks onto the GNS is shown in Fig. 5c, revealing that each microsphere is not a single crystal, but consists of ordered Fe₃O₄ nanoparticles with a diameter of about 10 nm. The selected area electron diffraction (SAED) pattern further confirms that each Fe₃O₄ microsphere is polycrystalline as shown in the bottom-right inset of Fig. 5c. The HRTEM image shows the typical lattice fringes of Fe₃O₄ particle in the GNS matrix (Fig. 5d). The lattice spacing of 0.296 nm and 0.253 nm correspond to the (220) and (311) planes of the Fe₃O₄ phase. These values are in good agreement with the standard data of Fe₃O₄ (JCPDS card no. 19-0629).

Fig. 4 SEM micrographs of (a) H-Fe₃O₄-MS/GNS composite, (b) bare Fe₃O₄.

Fig. 5 (a and b) Low- and high-magnification TEM micrographs of H-Fe₃O₄-MS/GNS composite, (c) a magnified image of a single H-Fe₃O₄-MS with a corresponding SAED pattern, and (d) a HRTEM image of H-Fe₃O₄-MS/GNS composite.

As reported previously, after a process of nucleation and oriented aggregation, hierarchical and porous Fe₃O₄ microspheres consisting of large numbers of Fe₃O₄ nanoparticles were formed under the solvothermal process.^55,56 In simple terms, the negatively charged GO sheets could absorb positively charged Fe³⁺ by the electrostatic attraction in the oxygen-containing groups, which served as the nucleation sites, so the primary magnetite nanocrystals first formed. Afterward, the newly formed nanocrystals aggregated into round spheres, driven by minimization of total surface energy. It is noted that the Fe₃O₄ microspheres are strongly anchored on GNS even after sonication for a long time during the preparation of the TEM specimen, accounting for the strong interaction between the Fe₃O₄ microspheres and GNS. Furthermore, it is believed that such interaction can improve the electrochemical performance, since the GNS can prevent the agglomeration of nanoparticles during charge–discharge process. In addition, the strong anchor of Fe₃O₄ nanoparticles on conductive GNS enables to accelerate Li ion and electron transport by providing a shortened diffusion pathway. Similarly, the nanoparticles anchored on the GNS can effectively prevent the restacking of GNS.⁵⁷

In view of the potential application as an anode material for LIBs, we investigated its ability of the H-Fe₃O₄-MS/GNS composite to reversibly react with Li ions. Fig. 6 presents the CV curves of H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ for the first three cycles in the potential range of 0–3.0 V (vs. Li/Li⁺) at a scan rate of 0.1 mV s⁻¹. As shown in Fig. 6a, a sharp reduction peak at about 0.6 V is observed in the first cathodic scan for the H-Fe₃O₄-MS/GNS composite, which can be attributed to the reduction of Fe³⁺ or Fe²⁺ to Fe⁰ and the irreversible reaction with the electrolyte.¹⁸ In addition, two weak reduction peaks at about 0.8 V and 1.54 V are observed for the composite, which may be ascribed to be the formation of Li_xFe₃O₄.⁵⁸ In this step, the conversion of Fe₃O₄ to Fe and the formation of amorphous Li₂O are the main reason of the irreversible capacity during the first discharge process. Meanwhile, two anodic peaks at about 1.63 and 1.81 V correspond to the reversible oxidation of Fe⁰ to Fe²⁺/Fe³⁺.⁵⁹ From the second cycle, both cathodic and anodic peaks are positively shifted in the subsequent cycles due to the structural modification after the first cycle. It is noted that the current density and the integrated area of the H-Fe₃O₄-MS/GNS composite are larger than those of the bare Fe₃O₄, and the peak at 1.81 V for the second or third cycle of the H-Fe₃O₄-MS/GNS composite which can be indexed to the reversible reaction to Fe₃O₄, are more obvious and intensive than those of the bare Fe₃O₄. Furthermore, there is no noticeable change of peak intensity and integrated areas for both cathodic and anodic peaks of the H-Fe₃O₄-MS/GNS composite after the first cycle compared with the bare Fe₃O₄, suggesting that the electrochemical reversibility of Fe₃O₄-graphene gradually establishes after the initial cycle and is much better than that for the bare Fe₃O₄. These all can be attributed to the fast Li ion and electron transport of strong covalent coupling interaction between the Fe₃O₄ microspheres and GNS. Furthermore, the porous structure of Fe₃O₄ microspheres can facilitate the infiltration of electrolyte which can also speed up the Li ion transport.

Fig. 6 CV curves of (a) H-Fe₃O₄-MS/GNS composite, (b) bare Fe₃O₄ for the first three cycles at a scan rate of 0.1 mV s⁻¹ in the potential range of 0–3.0 V (vs. Li/Li⁺).

Fig. 7 shows the first two galvanostatic charge–discharge curves of the H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ microsphere electrodes between 0.01 and 3.0 V at a current density of 200 mA g⁻¹. In the first discharge step, both curves present a long voltage plateau at about 0.75 V, followed by a sloping curve down to the cut voltage of 0.01 V, which are typical characteristics of voltage trends for the Fe₃O₄ electrode.^59,60 The second discharge curves of the H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ are both different from the first, suggesting the drastic, Li⁺-driven, structural or textural modifications.⁶¹ The first specific discharge capacity of H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ are 1336.4 and 1246.2 mA h g⁻¹, respectively. The extra capacity of the electrodes compared with the theoretic capacity resulted from the formation of solid electrolyte interface (SEI) film and possibly interfacial Li⁺ storage during the first discharge process.⁶² The polarization (i.e., voltage hysteresis between charge and discharge) could be due to the limited Li diffusion kinetics during the intercalation/deintercalation process.⁶³ As shown in Fig. 7, the H-Fe₃O₄-MS/GNS composite shows small voltage differences between the charge and discharge plateaus, indicating that the characteristics of strong covalent coupling possesses low electrochemical polarization and thereby induces good reversibility in the discharge–charge processes. Fig. 8 further presents the voltage hysteresis curve of the H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ electrodes at a current density of 200 mA g⁻¹. It is obtained by subtracting the discharge curve at the second cycle from the charge curve at the first cycle after normalization. For the capacity normalization used here, 0 refers to the full delithiation state charged to 3.0 V and 1.0 means the starting of charging.³ As shown in Fig. 8, the voltage polarization of bare Fe₃O₄ electrode is about 0.5–1.25 V in all lithiation/delithiation ranges, while the H-Fe₃O₄-MS/GNS composite exhibits an obvious reduction of voltage polarization to 0.3–1.1 V. It is considered that the decrease in voltage hysteresis for the H-Fe₃O₄-MS/GNS composite is attributed to the high electrical conductivity of GNS and the porous hierarchical structure of Fe₃O₄ microspheres, which accelerates the Li ion and electron transportation in the electrode.

Fig. 7 Charge–discharge profiles of H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ electrodes between 0.01 and 3.0 V at a current density of 200 mA g⁻¹.

Fig. 8 Comparison of voltage hysteresis curves of the H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ electrode at a current density of 200 mA g⁻¹.

Fig. 9a and b give the cycling performance of H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ at current densities of 200 and 500 mA g⁻¹, respectively. Obviously, the H-Fe₃O₄-MS/GNS composite exhibits a better cycling stability than the bare Fe₃O₄. The specific reversible capacities of the H-Fe₃O₄-MS/GNS composite after 70 cycles are 1171.6 mA h g⁻¹ at 200 mA g⁻¹ and 940.4 mA h g⁻¹ at 500 mA g⁻¹, much higher than those of the bare Fe₃O₄ (169 mA h g⁻¹ at 200 mA g⁻¹ and 68.7 mA h g⁻¹ at 500 mA g⁻¹). In addition, the reversible capacity of H-Fe₃O₄-MS/GNS composite increases gradually with the increase of cycle number at 200 mA g⁻¹, which can be observed in previous reports, such as graphene-wrapped Fe₃O₄ nanocomposite,^57,64 while the bare Fe₃O₄ presents a fast capacity fade, only can sustain 18.1% of the initial capacity. At the current density of 500 mA g⁻¹, the H-Fe₃O₄-MS/GNS composite still possesses an exceedingly high capacity retention of 96.4%, much higher than the bare Fe₃O₄ of 9.3%. The main reason for rapid capacity decay of the bare Fe₃O₄ electrode is believed that a large volume change and aggregation of particles occur during charge–discharge cycling. However, for the H-Fe₃O₄-MS/GNS composite electrode, the Fe₃O₄ microspheres are homogeneously decorated in the GNS. Such a dimensional confinement for the Fe₃O₄ microspheres by the surrounding GNS limits the volume expansion effect upon Li insertion. In addition, the GNS also provides a highly conductive network for electron transfer during the lithiation/delithiation process.

Fig. 9 Cycling performance of H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ at current densities: (a) 200 and (b) 500 mA g⁻¹.

The H-Fe₃O₄-MS/GNS composite also shows significantly enhanced rate performance compared with the bare Fe₃O₄, as shown in Fig. 10. With increasing the current density from 200 to 3000 mA g⁻¹, the discharge capacities of the two materials decrease, indicating the diffusion-controlled kinetics process for the electrode reaction.⁶⁵ But it can clearly observed that the discharge capacities of bare Fe₃O₄ electrode decrease drastically, the reversible capacity rapidly drops from 906.4 to 465.8 mA h g⁻¹ only in the first 10 cycles. The reversible discharge capacities of the H-Fe₃O₄-MS/GNS composite at 200, 500, 1000, 2000 and 3000 mA g⁻¹ are 1015, 992, 935, 852 and 745 mA h g⁻¹, respectively, greatly higher than those of the bare Fe₃O₄. When the current density is lowered down to 200 mA g⁻¹, the discharge capacity of the composite swiftly recovers to 1016 mA h g⁻¹ after 60 cycles, which demonstrates the excellent reversibility. To the best of our knowledge, the H-Fe₃O₄-MS/GNS composite synthesized in this method shows a superior rate retention. The improved cycling performance and rate capability are attributed to the high electrochemical activity and structure stability of the composite electrode.

Fig. 10 Rate performances of (a) H-Fe₃O₄-MS/GNS composite and (b) bare Fe₃O₄ electrodes.

The EIS measurements are conducted to verify that the GNS is responsible for the improved electrochemical performance of the H-Fe₃O₄-MS/GNS composite. Fig. 11a and b show the Nyquist plots of H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ after 3 and 70 cycles at a current density of 200 mA g⁻¹ measured at about 2.0 V. The shapes of the Nyquist plots of both electrodes are quite similar. The symbols are the experimental data whereas the continuous lines represent the fitted spectra. The Nyquist plots consist of a depressed semicircle where a high-frequency semicircle and a medium-frequency semicircle overlap each other and a long low-frequency line. The inclined line in the low-frequency region represents the Warburg impedance (Z_w), which is related to solid-state diffusion of Li⁺ in the electrode materials. The semicircle in the high frequency is assigned to the resistance (R_sl) of Li⁺ diffusion in the surface layer (SEI film). The semicircle in the middle frequency range indicates the charge-transfer resistance (R_ct), relating to charge transfer through the electrode/electrolyte interface. The intercept on the Z real axis in the high frequency region corresponds to the solution resistance (R_el).⁶⁵ In order to further understand the Nyquist plots and get well comparing of the plots obtained before and after the addition of GNS, a modified two-parallel diffusion path model is selected, and the corresponding equivalent circuit is presented in Fig. 11c, where R_el indicates the solution resistance; R_sl(i) and C_sl(i) stand for the resistance of migration and capacity of the surface-passivating layer, respectively; R_ct(i) and C_dl(i) designate the Li⁺ charge-transfer resistance and double-layer capacitance at the interface between electrolyte and electrode, respectively; and Z_W(i) represents the diffusion-controlled Warburg impedance (i = 1, 2).^20,65 The value of R_ct for H-Fe₃O₄-MS/GNS composite electrode through calculation increases from 3.21 Ω to 26.04 Ω after 70 cycles, whereas the one of the bare Fe₃O₄ increases strikingly, from 24.75 Ω to 175.28 Ω. But the values of R_sl for H-Fe₃O₄-MS/GNS composite and bare Fe₃O₄ electrodes all have no evident change after 70 cycles. Therefore, it is suggested that the charge-transfer resistance of the H-Fe₃O₄-MS/GNS composite is suppressed by the addition of GNS, which indicates that the electrons and Li ions can transfer more effectively between the interface of active materials and electrolyte, thus resulting in the enhanced electrode reaction kinetics during the charge–discharge process.

Fig. 11 Nyquist plots of (a) H-Fe₃O₄-MS/GNS composite, (b) bare Fe₃O₄ after 3 cycles (in circles) and 70 cycles (in squares) in the frequency range from 100 kHz to 0.01 Hz at 200 mA g⁻¹, and (c) the equivalent circuit.

Conclusions

In summary, we report a facile one-pot solvothermal route for preparation of a hierarchical Fe₃O₄ microsphere/graphene nanosheet (H-Fe₃O₄-MS/GNS) composite. The H-Fe₃O₄-MS/GNS composite exhibits a high specific capacity and a good cycling stability, reduced voltage hysteresis and enhanced rate capability. The improved electrochemical performance not only attributed to the GNS, which can accommodate volume expansion/contraction, avoid aggregation of Fe₃O₄ particles and improve the electrical conductivity of electrode, but also to the porous and hierarchical structure of Fe₃O₄ microspheres, which can facilitate Li ion transportation and accommodate volume expansion/contraction to some extent. The superior electrochemical properties of the H-Fe₃O₄-MS/GNS composite make it a promising anode material for advanced LIBs.

Acknowledgements

This work was supported by the Key Science and Technology Innovation Team of Zhejiang Province (2010R50013). The authors also thank the help of Dr Li'na Wang (Zhejiang Sci-Tech University).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45268a

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