Constructing aligned γ-Fe₂O₃ nanorods with internal void space anchored on reduced graphene oxide nanosheets for excellent lithium storage

Abstract

Unique 1D aligned γ-Fe₂O₃ nanorods anchored on 2D reduced graphene oxide nanosheets were successfully synthesized by a facile seed-assisted solution reaction and subsequent thermal decomposition method. The structural and compositional analyses by employing X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) show that each γ-Fe₂O₃ nanorod possesses internal void spaces. The special structural nature of the γ-Fe₂O₃ nanorods together with the hierarchical nanoarchitectures is responsible for a large specific surface area of ∼193.6 m² g⁻¹. When evaluated as an anode material for LIBs, the as-prepared composite electrodes delivered superior capacity, better cycling stability and rate capability. The excellent lithium storage properties are attributed to the advantageous structural features.

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1. Introduction

Rechargeable lithium-ion batteries (LIBs) with a series of advantages such as low self-discharge rate, high operating voltage, and no memory effect have been considered as one of the most promising candidates for applications in typical energy storage systems, for example, flexible electronics, mobile phones, laptop computers, and even electric vehicles, large-scale electric energy storage.^1–4 Developing advanced electrode materials is a key element to achieve the required performance and capacity of LIBs for the above important applications.^5,6 As the current commercial anode material, graphite cannot meet the requirements of high-performance LIBs in view of its low theoretical capacity (372 mA h g⁻¹) and serious safety problems that arise from low Li⁺ intercalation potential.^7,8 So far, alternative anode materials, such as transition-metal oxides,^9,10 sulfides,^11,12 nitrides,^13,14 alloys^15,16 and their composites have been exploited to exhibit better electrochemical performances.

Among those materials, transition metal oxides have received considerable attention since they can exhibit high capacity, widespread availability, and environmental benignity.¹⁷ Typically, iron oxides (such as Fe₃O₄ and Fe₂O₃) have been widely studied as anode materials for LIBs. They can electrochemically react with lithium through conversion mechanism to form metallic iron and Li₂O and deliver high theoretical capacities, for example, ∼920 and ∼1000 mA h g⁻¹ for Fe₃O₄ and Fe₂O₃, respectively. The reaction can be described as FeO_x + 2xLi⁺ + 2xe⁻ ↔ Fe + xLi₂O (x = 4/3 or 3/2).¹⁸ However, the large volume change and particle aggregation associated with the Li insertion and extraction processes are common problems during the practical use in LIBs, which often lead to electrode pulverization and loss of effective contact between the particles and in turn lead to the fast capacity fading and poor cycling stability. Such dynamical behaviors and conversion mechanism have been directly observed and demonstrated by using in situ transmission electron microscopy (TEM).¹⁹ Moreover, unsatisfactory electrochemical performance caused by kinetic limitations also impedes their applications.^20–22

To enhance the lithium storage properties of iron oxides, the construction of nanostructured materials with proper morphology (especially hollow nano/micro structures), composition, and microstructure has been shown to be an effective approach to overcome the above limitations, since the nanostructure facilitates electron and lithium ion transport, provides more active sites for the storage of lithium ions than its bulk counterpart, accommodates the local volume change upon charge/discharge cycling and is able to alleviate the problem of pulverization and aggregation of the electrode materials.^20–22 For example, monodisperse Fe₃O₄ and γ-Fe₂O₃ mesoporous microspheres were prepared via a surfactant-free solvothermal combined with precursor thermal transformation method. When evaluated as anode materials for LIBs, they showed a high initial discharge capacity of 1307 and 1453 mA h g⁻¹, respectively, and a good reversible performance (450 mA h g⁻¹ for Fe₃O₄ and 697 mA h g⁻¹ for γ-Fe₂O₃ after 110 cycles) at the current density of 0.2C (185 mA g⁻¹ for Fe₃O₄ and 201 mA g⁻¹ for γ-Fe₂O₃).²³ Fe₂O₃ nanospindles assembled with nanoparticles as primary building blocks, which synthesized in the choline chloride/urea mixture-based deep eutectic solvent system, show high capacity and good cycle stability (921.7 mA h g⁻¹ at a current density of 200 mA g⁻¹ up to 50 cycles), as well as the excellent rate capability.²⁴ Gu et al. reported a hierarchical structures composed by β-MnO₂ nanorods as a backbone and porous α-Fe₂O₃ nanorods as the branches. The branched nanorods deliver excellent reversible capacities and rate capabilities.²⁵ Wang and co-workers studied the effect of co-doping on the electrochemical performance, and found that the solid solution Co_xFe_3−xO₄ showed excellent cycle stability and rate performance.²⁶

On the other hand, the conductivity of typical transition-metal oxides is inferior to that of lithium metal and carbonaceous materials, which seriously restricts their applications in high-rate LIBs. Coating the electrochemically active materials with an electronically conductive agents layer (such as carbon nanofibers, carbon nanotubes, graphene, and so on) can not only enhance the conductivity of the electrode materials but also modify the chemistry at the electrode/electrolyte interface, which is considered as an effective approach to improve the cycling stability and rate capability.^17,27–34 Feng and Müllen et al. synthesised 3D graphene/Fe₃O₄ foams comprising interior graphene-encapsulated Fe₃O₄ nanosheets and exterior porous graphene networks. Superior cycling performance and excellent rate capability of the unique architectures were demonstrated.³⁵ Wang and Tu et al. reported a facile one-pot solvothermal route for the preparation of hierarchical Fe₃O₄ microsphere/graphene nanosheet composites, which exhibit a high specific capacity and a good cycling stability, reduced voltage hysteresis and enhanced rate capability.³⁶ Our previous works also show that low dimensional carbon nanostructures (such as carbon nanotube and graphene) based metal oxides and sulfides possess higher lithium storage capacity and better rate capability.^37–40

In this work, we report a facile seed-assisted solution reaction and subsequent thermal decomposition method for the construction of 1D aligned γ-Fe₂O₃ nanorods anchored on 2D reduced graphene oxide nanosheets. We found that each γ-Fe₂O₃ nanorod consists of internal void spaces in the designed 1D/2D nanoarchitectures (named as γ-Fe₂O₃ IVS-NRs/rGO nanocomposites thereafter), which results in a large specific surface area of ∼193.6 m² g⁻¹. Compared to the traditional simple hollow nanostructures, the internal void spaces in the γ-Fe₂O₃ nanorods provide more free room to accommodate the local volume change upon charge/discharge cycling, and sufficient active sites for the storage of lithium ions. Furthermore, the electronic conductivity of γ-Fe₂O₃ can be enhanced by the addition of rGO nanosheets. As a consequence, the yielded γ-Fe₂O₃ IVS-NRs/rGO nanocomposites exhibit a maximum specific capacity of 1284 mA h g⁻¹ at a current density of 0.1C (1C = 1000 mA g⁻¹) with outstanding charge/discharge rate capability (734 mA h g⁻¹ at 5C) and good cycling performance, demonstrating a great potential as anode materials for LIB applications.

2. Experimental

The rGO nanosheets used in this work were purchased from Nanjing XFNano Material Tech Co. Ltd. All the other analytical reagents were purchased from Tianjin Damao Chemical Reagent Co. Ltd, and used without further purification. In a typical synthesis, 0.5 g Fe(NO)₃·9H₂O was dissolved in 150 mL distilled water, then 75 mg rGO nanosheets was added to the solution. After sonication for 0.5 h, the mixture was stirred at 50 °C for 2 h. The precipitates were then separated by centrifugation, washed with distilled water and pure ethanol and then dried. Next, 125 mg of the precipitate was dispersed in 175 mL of a solution containing 2.5 g FeCl₃·6H₂O and 5.0 g NaNO₃, and this mixture were stirred for 0.5 h. The mixture was then transferred into a Teflon-lined stainless-steel autoclave for solution reaction at 60 °C for 12 h. After the autoclave cooled down to room temperature naturally, the precipitates were separated by centrifugation, washed with distilled water and pure ethanol and dried in air. The final products were yielded by thermal decomposition of the precursors at 300 °C for 2 h in air.

Crystallographic information for the samples was collected using a Bruker Model D8 Advanced powder X-ray diffractometer (XRD) Cu K_α irradiation (λ = 1.5418 Å). The morphology and microstructure of the products were examined using field-emission scanning electron microscopy (FESEM; Zeiss, MERLIN, 5 kV), TEM equipped with an energy dispersive X-ray (EDX) system (JEOL, JEM-2100, 200 kV), X-ray photoelectron spectroscopy (XPS, Escalab 250, Al Kα), and thermal gravimetric (TG) analysis (Netzsch-STA 449C, measured from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ under an air flow). The Barret–Joyner–Halender (BET) surface area of the powders was analyzed by nitrogen adsorption–desorption isotherm at 77 K in a Micromeritics ASAP 2010 system. The sample was degassed at 180 °C before nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method. A desorption isotherm was used to determine the pore size distribution via the Barret–Joyner–Halender (BJH) method, assuming a cylindrical pore model. The nitrogen adsorption volume at the relative pressure (P/P₀) of 0.994 was used to determine the pore volume and average pore size. Raman spectra were collected by using Raman microscopes (Renishaw, UK) under a 633 nm excitation.

The working electrodes were constructed by mixing the active materials, conductive carbon black and carboxymethyl cellulose, in a weight ratio of 80 : 10 : 10. The mixture was prepared as slurry and spread onto copper foil. The electrode was dried under vacuum at 120 °C for 5 h to remove the solvent before pressing. Then the electrodes were cut into disks (12 mm in diameter) and dried at 100 °C for 24 h in vacuum. The cells were assembled inside an Ar-filled glove box by using a lithium metal foil as the counter electrode and the reference electrode and microporous polypropylene as the separator. The electrolyte used was 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) with a volume ratio of EC/PC/DEC = 1 : 1 : 1. Assembled cells were allowed to soak overnight, and then electrochemical tests on a LAND battery testing unit were performed. Galvanostatic charge and discharge of the assembled cells were performed at a current density of 0.1C between voltage limits of 0.01 and 3.00 V (vs. Li⁺/Li) for 50 cycles. For the high rate testing, the discharge current gradually increased from 0.1C to 0.2, 0.5, 1 and 5C, and then decreased to 0.1C, step by step. All the charge/discharge testings were performed symmetrically at room temperature. The cyclic voltammogram (CV) was performed using a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai). CV curves were recorded between 0.01 and 3.00 V (vs. Li⁺/Li) at a scan rate of 0.5 mV s⁻¹. Electrochemical impedance spectroscopy (IM6, Zahner) was carried out by applying an AC voltage of 5 mV over a frequency range of 100 kHz to 0.01 Hz.

3. Results and discussion

The morphology of the as prepared precursors shows high density nanorods are anchored on 2D nanosheets (Fig. S1a and b†). Powder XRD result indicates except the diffraction peak at 2θ = 22.3° arises from the rGO nanosheets, all the other diffraction peaks can be indexed as β-FeOOH (JCPDS no. 34-1266) (Fig. S2†). The XRD indexation results are in good agreement with the selected area electron diffraction (SAED) pattern (Fig. S1c†). It should be pointed out here that the addition of NaNO₃ plays an important role in the formation of rod-like precursor. The NO₃⁻ ions serve as ligand to Fe³⁺, and may adsorb on the facets parallel to the c-axis of precursor nuclei by a monodentate structure to facilitate the relatively uniform growth of 1D nanorods. Similar results are also reported by others.^41–44 The final γ-Fe₂O₃ IVS-NRs/rGO nanocomposites can be obtained by thermal decomposition of the precursors at 300 °C for 2 h in air. The crystallographic structure and phase purity of the products are checked by XRD as shown in Fig. 1. All the diffraction peaks can be assigned to the planes of γ-Fe₂O₃ (JCPDS no. 39-1346, a = 8.3515 Å). No other diffraction peaks from possible impurities are observed, indicating the high phase purity of the products. Moreover, the relatively high peak intensities imply that the products are highly crystalline.

Fig. 1 XRD pattern of as-prepared sample and standard pattern of γ-Fe₂O₃ phase.

Typical low-magnification FESEM images of the final product shows that high density nanorods uniformly distributed on the substrate (Fig. 2a and b), which is similar to the precursors, implying the morphology is not destroyed in the thermal decomposition process. Higher magnification image (Fig. 2c) indicates that the nanorods are aligned on the surface of rGO nanosheets (the red arrows). Such 1D/2D nanoarchitectures are beneficial for preventing the agglomerating of rGO nanosheets and maintaining the large specific surface area. The detail structural information of the composites is studied by employing TEM and HRTEM characterizations. Fig. 2d and e show typical TEM images of γ-Fe₂O₃ IVS-NRs/rGO nanocomposites with different magnifications. γ-Fe₂O₃ nanorods are uniformly dispersed on the surface of rGO nanosheets, which are consistent with the FESEM observations. The average diameter and length of the nanorods are ∼11 nm and ∼60 nm, respectively. EDX analysis (Fig. S3†) shows that the nanorods are composed of Fe and O with an atomic ratio of ∼2 : 3, further confirming the formation of Fe₂O₃ (Cu single comes from the copper grid to support the sample for TEM observations). The SAED pattern indicates that the as obtained γ-Fe₂O₃ nanorods are polycrystal in nature (white arrows in Fig. 2f). The diffraction arcs marked with red arrow can be indexed as reflections of graphene, which is consistent with the known structure of rGO with disordered oxygen functional groups and random orientations.^45,46 By carefully investigating the microstructures details, we can find there are many internal void spaces in each γ-Fe₂O₃ nanorods (see white arrows in Fig. 2g). Since there are no obvious voids in the precursor nanorods (Fig. S1d†), the formation of internal void spaces can be ascribed to the decomposition of the precursors during the heat treatment process. Such IVS-NRs are different from the reported simple hollow nanostructures, such as nanotubes and hollow nanospheres, which possess only one cavity in it. The mutually independent internal void spaces in the nanorods can provide more free room to accommodate the local volume change during charge/discharge cycling process, sufficient active sites for the storage of lithium ions, and accelerate the mass diffusion and ion transport. Fig. 2h and i show typical HRTEM images of γ-Fe₂O₃ nanorods from the composites. The lattice spacings of d ∼ 3.78 Å, ∼2.14 Å and ∼4.88 Å are determined, which correspond to the (210), (040) and (422) planes of cubic γ-Fe₂O₃, respectively. The above results show that we succeeded in synthesizing γ-Fe₂O₃ IVS-NRs/rGO nanocomposites by a facile seed-assisted solution reaction and subsequent thermal decomposition method.

Fig. 2 (a–c) FESEM images, (d–i) TEM, HRTEM images and SAED pattern of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites. The red arrows in (c–g) show the existence of rGO nanosheets. The white arrows in (g) show the internal void space in γ-Fe₂O₃ nanorods.

The surface electronic states and chemical composition of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites are analyzed by using XPS in the region of 0–1350 eV. The survey spectra (Fig. 3a) shows obvious XPS peaks of iron, oxygen, and carbon, confirming the existence of Fe, O, and C elements in γ-Fe₂O₃ IVS-NRs/rGO nanocomposites, which is in agreement with previous EDS analysis. Fig. 3b shows the high-resolution XPS spectra of Fe 2p. The XPS peaks centered at ∼710.3 eV and ∼724.7 eV can be assigned to Fe 2p_3/2 and Fe 2p_1/2 level, respectively. In addition, a weak satellite peak appears at ∼719.0 eV, indicating that the iron oxide in the synthesized sample should be Fe₂O₃ rather than iron oxides with other compositions. The peak centered at ∼711.9 eV may be identified as the surface peak. The present results are in good agreement with the previous report.⁴⁷ Deconvolution of the O 1s peaks of the composites show four different kinds of oxygen related bonds (Fig. 3c). Of which the binding energy at ∼529.8 eV is due to the oxygen in γ-Fe₂O₃, and the peak centered at ∼531.5 eV can be ascribed to C=O in rGO.^48,49 From the fine spectrum of C 1s in the nanocomposites (Fig. 3d), four wide peaks are also seen at 284.5, 285.0, 286.2, and 288.9 eV, which are due to the C–C bond, non-oxygenated carbon, C=O bond (carbonyl), and O–C=O bond (carboxyl), respectively.^48,49

Fig. 3 (a) XPS survey spectra of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites, (b–d) high-resolution XPS spectra of the Fe 2p, O 1s, and C 1s regions, respectively.

The presence of graphene in the composites was further confirmed by Raman spectrum studies (Fig. S4†). Compared with the typical Raman features of commercial γ-Fe₂O₃ nanospheres (Aladdin Co. Ltd, 20 nm in diameter, Fig. S5†),²⁹ another two Raman peaks centered at 1371.4 and 1593.7 cm⁻¹ can be observed. The peak at 1371.4 cm⁻¹ (D band) is associated with the vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite from the defects and disorders of structures in carbon materials, while the peak at 1593.7 cm⁻¹ (G band) is attributed to the vibration of sp² hybridized C–C bond of an in-plane hexagonal lattice.⁵⁰ The I_D/I_G value is 0.72, indicating the formation of a reasonable degree of graphitization.^40,51 TG experiment was performed to determine the amount of rGO in the composites (Fig. S6†). The weight loss before 300 °C could be ascribed to surface water adsorption, while the weight loss after ∼300 °C could be ascribed to the oxidation of graphene in the nanocomposites, which yielding the weight fraction of rGO in the nanocomposites of about 19.2%. The presence of rGO in the composites could dramatically enhance the electronic conductivity of γ-Fe₂O₃ nanorods and is responsible for improving lithium storage ability.

The pore structures, including specific surface areas and the porous feature, of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites were studied by measuring nitrogen adsorption–desorption isotherms at 77 K (Fig. 4a). The specific surface area of the nanocomposites was calculated to be 193.6 m² g⁻¹ by using the BET method. In addition, the narrow mesopore size distribution based on the BJH method of the sample is further confirmed by the corresponding pore size distribution curve (Fig. 4b and the inset). The peak centered at ∼3.8 nm reveals that uniform mesopores dominate in the composites. The large surface area and narrow pore size distribution of the sample are due to the porous nature of the 1D/2D assembled nanoarchitectures and the amount of internal void spaces within each nanorods. The microstructure characteristics of the yielded γ-Fe₂O₃ IVS-NRs/rGO nanocomposites are favorable for using as anode materials in LIBs due to the capability of providing extra active sites for the storage of lithium ions, accommodating the local volume change during cycling, and facilitating mass diffusion and ion transport. The synthesized sample is therefore anticipated to show good lithium-storage properties.

Fig. 4 (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore size distribution curve the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites.

The electrochemical performance of γ-Fe₂O₃ IVS-NRs/rGO nanocomposites as anode materials for LIBs was measured. The electrochemical behavior of the assembled cells is first investigated by CV experiments between 0.01 and 3 V vs. Li⁺/Li at a scan rate of 0.5 mV s⁻¹. Fig. 5a shows the CV curves of γ-Fe₂O₃ IVS-NRs/rGO nanocomposites for the first three cycles at a scan rate of 0.5 mV s⁻¹ in the range from 0.01 to 3.00 V (vs. Li⁺/Li). In the first cycle, the cathodic peak at 0.60 V is due to the formation of solid electrolyte interface (SEI) layer on the electrode surface and Li₂O, which is induced by the irreversible decomposition reaction of electrolyte.⁵² The anodic peaks at 1.68 V and 1.93 V are attributed to the reversible oxidation from Fe⁰ to Fe³⁺.⁵³ During the subsequent cycles, the reduction peak potential shifts to ∼0.8 V while the oxidation peak position is almost the same. It may attribute to the reduced polarization induced by small size of the electrogenerated nanograins. The CV results are in good agreement with the previous reports.^17,24 The shape of CV curves in the following cycles remains similar to that of the second cycle, indicating high electrochemical reversibility of lithium storage of the composite electrode. The charge–discharge voltage profiles of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites for the first cycle at a current rate of 0.1C is shown in Fig. S7.† The results for commercial γ-Fe₂O₃ nanopowders are also displayed for comparison. In the first discharge curve, there is a dominant potential plateau (∼0.89 V) and a following slope, which represents lithium ion insertion process, and the formation of SEI film, respectively. The initial discharge and charge capacities of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites are 1784.5 and 1305.4 mA h g⁻¹, yielding the coulombic efficiency value of 73.2%. In contrast, the first cycle discharge and charge capacities of the γ-Fe₂O₃ nanopowders are 1080.6 and 816.9 mA h g⁻¹. Such initial irreversible capacity loss should mainly originate the formation of SEI layer due to the irreversible degradation of electrolyte and other secondary reactions, which is common for iron oxides materials.^17,23–26Fig. 5b shows the charge–discharge cycling performance together with the coulombic efficiency at a current density of 0.1C between 0.01 and 3 V vs. Li⁺/Li. It is obvious that the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites electrode exhibits a much better cycling performance than γ-Fe₂O₃ nanopowders electrode. The as-prepared γ-Fe₂O₃ IVS-NRs/rGO nanocomposites exhibit excellent cyclic capacity retention from the second cycle onwards. A reversible capacity 1284 mA h g⁻¹ can be retained at the end of 50 charge–discharge cycles. For comparison, the reversible capacity of γ-Fe₂O₃ nanopowders decreases to 550 mA h g⁻¹ after 50 cycles of operation under the same experimental conditions. As rate capability is an important parameter for LIBs applications, we also investigated the electrochemical performance of the composites at various rates ranging from 0.1C to 5C as shown in Fig. 5c. The reversible capacities of γ-Fe₂O₃ nanopowders tested at 0.1C, 0.2C, 0.5C, 1C, and 5C are 880.2 mA h g⁻¹, 800.8 mA h g⁻¹, 710 mA h g⁻¹, 452 mA h g⁻¹, and 80.8 mA h g⁻¹, respectively. Correspondingly, the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites deliver reversible capacity of 1282.2 mA h g⁻¹ at 0.1C, 1124.9 mA h g⁻¹ at 0.2C, 1056.2 mA h g⁻¹ at 0.5C, 986.3 mA h g⁻¹ at 1C, and 734 mA h g⁻¹ even at 5C. The capacity of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites can recover to the initial value as long as the current density reverses back to a low rate. To understand the excellent electrochemical performance of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites, the morphology change of the composites after cycling testing (50 cycles) were examined using SEM and TEM (Fig. S8†). It is obvious that the morphology and size of γ-Fe₂O₃ IVS-NRs are almost the same as in their initial state, which uniformly disperse on the surface of rGO nanosheets. The results show that the good structural and morphological stabilities of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites during charge/discharge cycling. The electrochemical impedance measurements are carried out to determine the Li⁺ transfer behavior. Fig. 5d shows the impedance spectra for γ-Fe₂O₃ IVS-NRs/rGO nanocomposites and γ-Fe₂O₃ nanopowders. In the impedance spectrum, the inclined lines (low-frequency) correspond to the Li diffusion process inside the electrode material, the medium-frequency semicircle is related to the charge-transfer resistance on electrolyte and the electrode interface, and the high frequency semicircle is attributed to the contact resistance occurring because of the SEI film.^54,55 The Nyquist plots in the frequency range from 100 kHz to 0.01 Hz clearly show that the diameter of the semicircle of γ-Fe₂O₃ IVS-NRs/rGO nanocomposites is much smaller than that of γ-Fe₂O₃ nanopowders, indicating that the charge transfer process is enhanced for γ-Fe₂O₃ IVS-NRs/rGO nanocomposites, which is beneficial for improving rate capability.

Fig. 5 Electrochemical performance of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites: (a) CVs at a scan rate of 0.5 mV s⁻¹ between 0.01 and 3 V, (b) cycling performance of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites (blue lines) and commercial γ-Fe₂O₃ nanopowders (black lines) electrodes, (c) rate capability of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites (blue lines) and commercial γ-Fe₂O₃ nanopowders (black lines) electrodes at various current rates between 0.1C and 5C, and (d) Nyquist plots of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites (blue lines) and commercial γ-Fe₂O₃ nanopowders (black lines) electrodes measured with an amplitude of 5 mV over the frequency range of 100 kHz and 0.01 Hz. The inset shows the equivalent electrical circuit. R_e is the electrolyte resistance, R_ct is the charge-transfer resistance, Z_w is the Warburg impedance related to the diffusion of Li ions into the bulk electrodes, and CPE is the constant phase-angle element, involving double layer capacitance, R_SEI and C_SEI are the pseudocapacitance and resistance of SEI film, respectively.

The excellent lithium storage properties should be related to the advantageous structural features of the γ-Fe₂O₃ IVS-NRs/rGO nanocomposites. Specifically, the unique γ-Fe₂O₃ nanorods with internal void spaces provide more free room to accommodate the local volume change upon charge/discharge cycling.⁵⁶ rGO nanosheets in the composite also give an elastic buffer space to accommodate the volume expansion/contraction of γ-Fe₂O₃ NRs. At the same time, the 1D/2D nanoarchitectures are favored for preventing the aggregation of the nano/microcrystals. The above factors are of importance for the cycling stability of the electrodes. The large specific surface area (∼193.6 m² g⁻¹) of the nanocomposites results from the internal void spaces in γ-Fe₂O₃ nanorods and hierarchical structural nature can provide extra active sites for the storage of lithium ions, which is beneficial for increasing the specific capacity. Last but not the least, the addition of rGO nanosheets and the creation of internal void spaces in γ-Fe₂O₃ nanorods can reduce the effective distance for lithium ions and electrons transport, and mass diffusion, which are of importance to obtain high rate capabilities.^33–36

4. Conclusions

In summary, we succeeding in constructing unique 1D aligned γ-Fe₂O₃ nanorods anchored on 2D reduced graphene oxide nanosheets by a facile seed-assisted solution reaction and subsequent thermal decomposition method. Structural characterizations show that each γ-Fe₂O₃ nanorod consists of internal void spaces in the 1D/2D nanoarchitectures, which results in a large specific surface area of ∼193.6 m² g⁻¹. When used as the anode materials for LIB, the yielded γ-Fe₂O₃ IVS-NRs/rGO nanocomposites delivered good lithium storage properties in terms of cycling stability, specific capacity, and rate capability. The composites exhibit a maximum specific capacity of 1284 mA h g⁻¹ at a current density of 0.1C with outstanding charge/discharge rate capability (734 mA h g⁻¹ at 5C) and good cycling performance. Therefore, γ-Fe₂O₃ IVS-NRs/rGO nanocomposites can be considered to be an attractive candidate as an anode material for LIBs.

Acknowledgements

This research is supported by Chinese National Natural Science Foundation (51401114) and China Postdoctoral Science Foundation (2013M542528, 20110490024). This work made use of the resources of the Beijing National Center for Electron Microscopy.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, additional SEM, TEM images and XRD, TGA curves. See DOI: 10.1039/c5ra16671c

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