A corn-inspired structure design for an iron oxide fiber/reduced graphene oxide composite as a high-performance anode material for Li-ion batteries

Abstract

In this paper, we successfully synthesized iron oxide (Fe₂O₃) fiber/reduced graphene oxide (rGO) composites with a “corn” structure by an electrospinning technique assisted by annealing treatment and a far infrared reduction process. The special structure consists of Fe₂O₃ fibers as the “corncob” completely protected by multilayer rGO as the “sepal”. Natural void space between the Fe₂O₃ fibers and rGO allows for the expansion of Fe₂O₃ upon lithiation; the good surface area and unblocked channels in the Fe₂O₃ fiber facilitates fast diffusion of Li⁺. An electrode with such structure shows excellent capacity (1085.2 mA h g⁻¹ at 0.1 A g⁻¹), and cycle life (407.8 mA h g⁻¹ at 5 A g⁻¹ for 1500 cycles with good coulombic efficiency). This is the longest cycle life for Fe₂O₃-based anode materials with excellent rate capability (e.g., >400 mA h g⁻¹ at 5 A h g⁻¹). In addition to Fe₂O₃, this “corn” structure can also be applied to other high capacity anode materials for next generation Li-ion batteries to improve cycle life and coulombic efficiency.

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1. Introduction

Recently, lithium-ion batteries (LIBs) have attracted much attention due to the rapid development of portable electronic devices, electric vehicles (EVs), and hybrid electric vehicles (HEVs).^1–3 At present, most commonly used anode materials for commercial LIBs are based on graphite. However, graphite suffers from an inherently low theoretical charge storage capacity (372 mA h g⁻¹) and often operates at significantly low rates (typically, <1C) that in turn limits the achievable power densities.^4,5 Therefore, the design and synthesis of new anode materials is necessary to offer the promise of high performance lithium-ion batteries with high efficiency, great energy density and long-lasting duration to meet various energy storage demands. For the purpose, nanostructure transition metal oxides have received great interest in lithium-ion batteries, and scientists have made great efforts to explore alternative anode materials with high capacity from them, such as SnO₂, FeO_x, CoO_x, MnO_x, CoFe₂O₄, and Co₃V₂O₈.^6–13 Among these anode materials investigated for LIBs, Fe₂O₃ have been widely studied as anode materials due to their high theoretical capacity value (1070 mA h g⁻¹), low cost, natural abundance, nontoxicity and the highest electrical conductivity (∼2 × 10⁻⁴ S m⁻¹) than other metal oxides.^14–16

However, it remains great challenges in the application of Fe₂O₃ to commercial LIBs, especially the obstacles of poor cycling stability and inferior rate capability. The large volume expansion and severe collapse occurs in the host matrix of Fe₂O₃ during the cycling processes give rise to pulverization, which results in the breakdown of electrical connection of these anode materials from current collectors and rapid capacity fading upon cycling. To overcome these drawbacks, several strategies have been employed such as Fe₂O₃ nanotubes, nanorods/rGO, nanoropes/rGO, hollow nanobarrels, hollow nanoparticles/rGO, hollow spheres, and hollow-structured tubular nanostructures.^17–28 Chaudhari et al.²⁶ reported a hollow-structured α-Fe₂O₃ nanofibers electrode with a reversible capacity of 1293 mA h g⁻¹ at a current density of 60 mA g⁻¹. Fe₂O₃ hollow nanoparticles/N-doped graphene aerogels were prepared by Liu et al.,²⁷ and the grapheme/Fe₂O₃ aerogels exhibit high rate capability and excellent cyclic stability (729 mA h g⁻¹ at 0.1 A g⁻¹ for 300 cycles). Zhu et al.²⁸ reported the graphene oxide/Fe₂O₃ composite exhibiting 1027 mA h g⁻¹ at a current density of 100 mA h g⁻¹ after 50 cycles. These results suggested that incorporating nanoscale iron oxide into rGO has a great beneficial impact on the electrochemical performance of Li-ion batteries.

More recently, some core–shell nanostructure materials, which was originally used in semiconductors, have been imported to the field of Li-ion batteries. Some efforts was made to investigate Li-ion battery materials with core–shell nanostructure,^29–33 such as Si, alloys, and transition metal oxides with carbon shells, and graphene was also introduced as a shell material. For example, Yang et al. have been fabricated graphene-wrapped metal oxides particles by electrostatic self-assembling. This unique core–shell structure have suppressed the aggregation of oxide nanoparticles, accommodated the volume change during the discharge–charge processes, and keep the high electrical conductivity of the electrode. The obtained graphene-wrapped Co₃O₄ hybrids delivered very high reversible capacity of about 1100 mA h g⁻¹ in the initial 10 cycles and 1000 mA h g⁻¹ after 130 cycles at 74 mA g⁻¹. However, further exploration and rational fabrication of iron oxide composite electrode materials with high-rate performance and long cycle life will have a significant impact on its practical utilization in Li-ion batteries.

Herein, we fabricated a novel core–shell structure Fe₂O₃ fiber/rGO anode materials by electrostatic self-assembling and the negatively charged graphene layers was wrapped on the positively charged Fe₂O₃ nanofiber. As shown in Fig. 1, the core–shell structure of Fe₂O₃ fiber/rGO composite is very similar with that of corn; Fe₂O₃ nanofiber can be regarded as the “corncob”, and the wrapping multilayer reduced graphene oxide as the “sepal”. This unique composites with corn-like structure has several advantages for LIB anodes. First, the rGO layers are a stable framework, and the void space between the Fe₂O₃ fiber and the rGO layers allows for the Fe₂O₃ to expand upon lithiation. Second, the rGO layers prevent the electrolyte from reaching the Fe₂O₃ surface inside the rGO layers completely, and lithiation of the Fe₂O₃ occurs by Li diffusion through the rGO into the Fe₂O₃ fiber. Third, the rGO layers are both electronically and ionically conducting, which allows for good kinetics in Fe₂O₃ fiber/rGO anode. Due to the cohesiveness and good conductivity of rGO, Fe₂O₃/rGO anode is applied for LIBs without binder and conductive agent as dopant. As a result, the synergistic effect between rGO and Fe₂O₃ fiber could reduce the initial capacity loss as well as improve the high rate capability (1085.2 mA h g⁻¹ at 0.1 A g⁻¹) and cyclability (407.8 mA h g⁻¹ at 5 A g⁻¹ for 1500 cycles). This is the longest cycle life for the Fe₂O₃-based anode materials with excellent rate capability (e.g., >400 mA h g⁻¹ at 5 A h g⁻¹).

Fig. 1 Schematic of the materials design.

2. Experimental section

2.1 Preparation of Fe₂O₃ fibers

Fe₂O₃ fibers were synthesized by electrospinning technique assisted with anneal treatment. In a typical experiment, 2.4 g poly(vinyl pyrrolidone) (PVP, M_w = 1 300 000, Sigma) was dissolved in 15 ml N,N-dimethylformamide (DMF, AR, Sigma) and 15 ml ethanol solution, magnetic stirring for 3 h. Then, 2.4, 4.8, or 7.2 g ferric chloride (AR, Aladdin, China) was added the solution and then vigorous stirring for 2 h, and FeCl₃ concentrations to the whole solution are 6, 12 and 18 wt%, respectively. The as-prepared samples were designated as PVP/FeCl₃ (I), PVP/FeCl₃ (II) and PVP/FeCl₃ (III), respectively. Next, the precursor solution was loaded into a plastic syringe that was equipped with a stainless steel needle of 0.8 mm in diameter and electrospun at a DC voltage of 15 kV and a flow rate of 0.6 ml h⁻¹. The electrospun fibers were collected on aluminum foil and the distance between the needle and collector was 15 cm. The as-electrospun PVP/FeCl₃ composite fibers were placed in a vacuum oven for 12 h at room temperature in order to remove the solvent residuals, and calcined for 3 h in air at 600 °C (the heating rate, 1 °C min⁻¹) in a tube furnace, respectively. Finally the Fe₂O₃ fibers obtained were designated as Fe₂O₃ (I), Fe₂O₃ (II) and Fe₂O₃ (III), respectively.

2.2 Preparation of Fe₂O₃/rGO composites

Graphene oxide was synthesized from natural graphite by a modified Hummers method described in detail previously.³⁴ To prepare Fe₂O₃/rGO composites, a homogeneous suspension of graphene oxide (GO) and Fe₂O₃ fibers was carefully prepared. A total of 20 ml of alcoholic GO (1 mg ml⁻¹) was sonicated for 30 min to form a stable solution. Subsequently, 0.01 g Fe₂O₃ fibers was mixed with alcoholic GO and sonicated for another 30 min to form a homogeneous dispersion. Standing for 24 h, the Fe₂O₃/GO composites was collected by centrifugalization. The collecting solid was dried for 24 h by vacuum at 60 °C. Finally, the Fe₂O₃/GO composites was put in home-use convection oven and irradiated by far infrared light for 5 min, in which the mixture color changed from bright red to dark red. Then Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO Fe₂O₃/rGO composites were obtained.

2.3 Characterization

The morphology and diameter of samples were characterized using scanning electron microscope (SEM, FEI Quanta200F) and transmission electron microscope (TEM, FEI Tecnai G2 F20, 200 kV). The crystalline structural characterization of the all samples was carried out by X-ray diffraction (XRD) using a diffractometer (Bruker D8 Focus X-ray diffractometer), and the diffraction patterns were recorded at room temperature in the 2θ range between 5° and 80°. The chemical structure of fibers was conducted with a Fourier transform infrared (FT-IR) reflection spectroscopy (NICOLET 380, Wisconsin, USA). Weight loss behavior was tested by thermogravimetric (TG) analysis (TGA/DSC 3+, Mettler Toledo, Zurich, Switzerland) (air, 10 °C min⁻¹).

2.4 Electrochemical performance evaluation

The electrochemical performance of the Fe₂O₃ fibers and Fe₂O₃/rGO composites were analyzed by constructing a 2025-type coin cell. The active material and a certain amount of EtOH were mixed to prepare slurry, and the resulting slurry was attached onto copper foils. The anodes were dried in a vacuum furnace at 65 °C for 24 h. The mass loading of active materials per electrode is about 0.4 mg cm⁻², in which the mass of Fe₂O₃/rGO composites for the working electrodes was obtained by a Sartorius scale (0.01 mg) and the area of copper sheets were about 2 cm². The cells were assembled in a high purity argon atmosphere inside a glove box (Mbraun lab Master130, Germany). Lithium ribbon (0.3 mm thick) was used as the counter electrode and Celgard® 2325 was used as the separator and the electrolyte was a solution of 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC) : dimethyl carbonate (DMC) (1 : 1 vol%). Cyclic voltammetry (CV) measurements and electrochemical impedance spectroscopy (EIS) were performed using electrochemical workstation (PARSTAT 2273, Prinston, USA) at 0.1 mV s⁻¹ scan rate between 0.0 and 3.0 V. Charge and discharge were conducted using an a multi-channel battery test system (NEWARE, Shenzhen, China) at several different current densities between cutoff potentials of 0.02 and 3.00 V. The capacities were calculated on the basis of the weight of Fe₂O₃ fibers or Fe₂O₃/rGO composites.

3. Results and discussion

The fabrication process of the flexible Fe₂O₃ fiber/rGO composite is illustrated in Fig. 2 and S1 (ESI†). Typically, a homogeneous viscous solutions with FeCl₃ dissolved in poly(vinyl pyrrolidone) was electrospun into fibers, following with annealing process at 600 °C in air. Then, the Fe₂O₃ fibers were sonicated with alcoholic GO solution and formed a homogeneous dispersion; during the process, the negatively charged graphene oxide was uniformly wrapped on positively charged Fe₂O₃ nanofiber. In the next step, the Fe₂O₃/GO composites were collected by centrifugalization. Finally, the Fe₂O₃/GO composites was reduced to Fe₂O₃/rGO by the thermal decomposition of graphene oxide (GO) in air at a relatively low temperature of ∼200 °C in under 5 minutes through far infrared (FIR) irradiation via a home-used convection oven.

Fig. 2 Schematic diagram for the fabrication of Fe₂O₃/rGO composites.

To understand the influence of FeCl₃ concentration on the PVP/FeCl₃ fibers, three different weights PVP to FeCl₃ in DMF solution (FeCl₃ contents of 6, 12 and 18 wt%, respectively) were applied by electrospinning processes. The as-prepared fibers were designated as PVP/FeCl₃ (I), PVP/FeCl₃ (II) and PVP/FeCl₃ (III), respectively. Fig. 3 shows the SEM images of PVP/FeCl₃ fibers with different FeCl₃ contents. All electronspinning PVP/FeCl₃ fibers are straight and exhibit an interconnected pore structure and homogeneously distributed diameters. The fiber diameter increases from ∼400 nm to ∼1500 nm with the increasing of the contents of FeCl₃ increased from 6 to 18 wt%. However, PVP/FeCl₃ (III) shows junction between single fiber and the larger size. This phenomenon can be explained that the fibers have a lower viscosity at higher concentration of FeCl₃ which results in the junctions morphology.³⁵

Fig. 3 SEM images of (a and d) PVP/FeCl₃ (I), (b and e) PVP/FeCl₃ (II) and (c and f) PVP/FeCl₃ (III).

To obtain the α-Fe₂O₃ fibers with good crystallinity, the FTIR spectra and X-ray diffraction spectra have been analyzed by calcining under different temperatures. Fig. 4a shows the FTIR spectra of the PVP/FeCl₃ fibers and calcined in air at 200–800 °C for 3 h. There are two peaks appearing at 463 and 551 cm⁻¹ which are assigned to the Fe–O vibration of the Fe₂O₃ fibers from 400 to 800 °C. Due to the decomposition of PVP, the peaks at about 1621 cm⁻¹ and 3369 cm⁻¹, corresponding to –N–C(=O)–, –O–H, respectively, disappear upon calcination at 600 °C, which indicates the formation of pure Fe₂O₃. Fig. 4b shows the X-ray diffraction patterns of the PVP/FeCl₃ fibers calcined in air at 200–800 °C for 3 h. The observed diffraction peaks at 2θ (°) = 24.14, 33.16, 35.62, 40.86, 49.46, 54.07, 57.61, 62.43, 63.99, 71.96 and 75.45 correspond to the planes of (012), (104), (110), (113), (024), (116), (018), (214), (300), (1010) and (220), respectively, which all match with the rhombohedral phase of α-Fe₂O₃ (JCPDS 33-0664).^26,27 The sharp diffraction peaks clearly indicate the good crystallinity of the α-Fe₂O₃ phase. The diffraction peaks of the Fe₂O₃ fibers were observed above 400 °C in sintering processes, and enhanced upon calcinations at 600 °C, which was in accordance with the FTIR results.

Fig. 4 (a) FTIR spectra of PVP/FeCl₃ (II) fibers and calcined in air at 200–800 °C; (b) X-ray diffraction patterns of the PVP/FeCl₃ (II) fibers calcined in air at 200–800 °C for 3 h.

Fig. 5 show the SEM and TEM images of Fe₂O₃ (I), Fe₂O₃ (II) and Fe₂O₃ (III) after heat treatment at 600 °C. As can be seen from the figures, the average diameter of these fibers greatly shrank because of the weight loss resulting from the removal of PVP after heat treatment. The average diameters of Fe₂O₃ (I), Fe₂O₃ (II) and Fe₂O₃ (III) fibers were found to be approximately 200, 400, 800 nm, respectively. It is obvious in Fig. 5 that corncob-like fibers were formed through the above approach and their surface became rough and porous due to some decomposition of the organic components.

Fig. 5 SEM and TEM images of (a, d and g) Fe₂O₃ (I), (b, e and h) Fe₂O₃ (II) and (c, f and i) Fe₂O₃ (III).

Based on these Fe₂O₃ fibers obtained, the Fe₂O₃/rGO composites have been prepared after a far infrared (FIR) irradiation process. To prove the reduction effect of GO in the composite, the Raman spectrum of the Fe₂O₃/rGO samples before and after reduction was shown in Fig. S2.† It is found that two broad peaks locate at 1345 and 1589 cm⁻¹, which correspond to the A_1g vibration mode of the disordered carbon (D-bond) and the E_2g vibration mode of the ordered graphitic carbon (G-bond), respectively. The intensity ratio value of I_D/I_G for the samples increase from 0.92 to 0.97, which shows the graphene oxide has been reduced to rGO by far infrared irradiation reduction process. The higher I_D/I_G value indicates more defects and edges in the reduced graphene structure which may results in more porous structure. Interestingly, the D peaks of in the nanocomposites show the slightly red-shift, revealing the stronger interactions between Fe₂O₃ nanofiber and graphene after reduction process. This phenomenon usually derives from the dielectric confinement effect of transition metal oxide on graphene.^36,37 Further, X-ray photoelectron spectroscopy (XPS) measurements also provide direct evidence of the reduction of GO under FIR irradiation and chemical state of Fe₂O₃ in composites, as shown in Fig. S3.† The survey scan spectrum displays the peaks of iron, oxygen and carbon, showing a high purity of as-prepared Fe₂O₃/rGO composites. The C 1s XPS spectra of Fe₂O₃/rGO composites indicates that, for Fe₂O₃/GO samples, four different peaks centered at 285.0, 286.8, and 288.1 eV were observed, corresponding to the presence of C–C/C=C in aromatic rings, C–O (epoxy and alkoxy), C=O groups, respectively. After FIR irradiation reduction treatment, the intensities of all the C 1s peaks of the carbons binding to oxygen, decrease obviously. It suggests that the GO have been reduced to rGO in a certain degree. And for the XPS spectra of the Fe 2p for Fe₂O₃/GO composites, binding energies of Fe 2p_3/2 and Fe 2p_1/2 are 711.4 eV and 724.9 eV respectively for Fe₂O₃/GO composites before and after reduction, in good agreement with crystalline Fe₂O₃. The satellite peak locating at 719.9 eV between the Fe 2p_3/2 and 2p_1/2 peaks was assigned to its purely trivalent nature, implying that the oxidation state of Fe element in the composites were of Fe³⁺ only.³⁸ Furthermore, the weight content of reduced graphene oxide can be obtained by comparing with the thermal weight loss behaviors between pure Fe₂O₃ fibers and Fe₂O₃/rGO composites, as shown in Fig. S4.† For the sample of Fe₂O₃ fibers, it can be seen that the weight losses could be attributed to the evaporation of moisture and crystal water below 200 °C in the curves. With the temperature increasing, there is no weight loss for the sample of pure Fe₂O₃ fibers, which shows that Fe₂O₃ crystalline phase has good thermal stability. Instead, there are significant weight loss above 200 °C in the curve for the samples of Fe₂O₃ fibers/rGO composites: the weight loss between 200 to 300 °C corresponds to the further reduction process of reduced graphene oxide; the weight loss between 300 °C and 500 °C could be attributed to the complete combustion of graphene; and no further weight loss above 500 °C indicated the pure Fe₂O₃. Therefore, the weight percentage of Fe₂O₃ in the composite samples can be estimated by about 50%.

To further investigate the morphological structure of the as-prepared Fe₂O₃/rGO composites, the SEM and TEM images were shown in Fig. 6. A typical morphology of Fe₂O₃/rGO hybrid fibers and porous structure have been clearly observed in Fig. 6a and b. The elemental mapping (Fig. 6c) showed that the Fe₂O₃/rGO composites were uniformly distributed with carbon, oxygen, and iron. Fig. 6d demonstrates Fe₂O₃ fiber was uniformly coated by graphene oxide, which looks like a corn, even via a reduction of far infrared light irradiation (Fig. 6e). It is obvious that the surface of Fe₂O₃/rGO composites has a smooth film, which indicates Fe₂O₃ was coated in the rGO sheets. This phenomenon can be explained as follows: the oxygen-containing functional groups (hydroxyl, carbonyl and carboxyl groups) on the GO make it negatively-charged and they can act as anchor sites that interact strongly with the covered species;³⁹ when treating the mix solution of negatively charged GO and positively charged Fe₂O₃ fibers with ultrasound, the positively-charged Fe₂O₃ fiber may be absorbed on the negatively-charged surface of GO, which may promote the coating of Fe₂O₃ fibers by GO. A regular diffraction spot ring is observed from the selected-area electron diffraction (SAED) pattern (Fig. 6d and e, inset). The SAED pattern of rGO (Fig. 6d) shows the ring pattern obviously, and Fig. 6e shows the typical hexagonally ordered lattice of carbon in graphene, which indicated the Fe₂O₃/GO was transformed into Fe₂O₃/rGO completely. In addition, the reduction effect is also confirmed by XRD patterns (Fig. 6f). As shown in Fig. 6f, the sharp X-ray diffraction peak of GO (at 2θ ≈ 10°) appears in Fe₂O₃/GO composites before far infrared irradiation, and after far infrared irradiation, the composites obtained appear the new diffraction peak of rGO (at 2θ ≈ 24°) and disappear the peak of GO (at 2θ ≈ 10°), indicating the Fe₂O₃/GO was transformed into Fe₂O₃/rGO completely. In the meantime, crystallined α-Fe₂O₃ and graphene layers are also confirmed by XRD patterns (Fig. 6f) and HRTEM (Fig. S5†). As estimated by the HRTEM images, the lattices with a lattice fringe spacing of 0.22 nm are indexed to the (113) plane of α-Fe₂O₃, and the thickness of rGO layer is about 3 nm.

Fig. 6 (a and b) SEM images and (c) element mapping patterns of Fe₂O₃/rGO composites; (d and e) TEM and SAED images of Fe₂O₃/GO composites and Fe₂O₃/rGO composites; (f) X-ray diffraction patterns of the Fe₂O₃/GO composites and Fe₂O₃/rGO composites after far infrared light irradiation.

Subsequently, the porous characteristics of the Fe₂O₃ fiber/rGO composite were investigated by the nitrogen isothermal adsorption–desorption measurement. As displayed in Fig. S6,† the pore size distribution of the Fe₂O₃ fiber/rGO composite is almost consistent with pure Fe₂O₃ nanofiber, which give the evidence that the inside channels in Fe₂O₃ fiber remained unblocked after graphene sheets wrapping. The BET specific surface area of Fe₂O₃ fiber/rGO composites was estimated as 150.9 m² g⁻¹ from the result of nitrogen adsorption data, and a more than 43% increase in specific surface area is found by comparing with pure Fe₂O₃ nanofiber. The major increase of the specific surface area can be attributed to the wrapped graphene nanosheets with relatively high specific surface area. It indicates that the graphene sheets working as a robust support can effectively suppress the self-aggregation of Fe₂O₃ nanofiber and thus further maintain the surface area of the Fe₂O₃ fiber/rGO material. At the same time, the synergistic effect of the unblocked channels and good surface area in the Fe₂O₃ fiber/rGO composite will facilitate fast diffusion of Li⁺ when large area of active sites was provided for the lithiation and delithiation reactions.

Therefore, the as-prepared “corn” structure materials of Fe₂O₃ fiber/rGO composites with Fe₂O₃ fiber as the “corncob” and multilayer reduced graphene oxide as the “sepal” will be expected to relieve the pulverization originated from the large volume change during the charge/discharge cycles, and further improve the electronic conductivity of Fe₂O₃ composites electrode, which contribute to high rate capability, and superior cyclability as Li-ion anode.

To investigate the possibility of the Fe₂O₃/rGO composites used as advanced anode materials, the coin-type Li half-cells has been assembled and their electrochemical performance were studied. The initial cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s⁻¹ for the first five cycles were shown in Fig. 7a. In the first scan, irreversible reduction peaks were observed at 1.57, 0.90 and 0.60–0.75 V for the Fe₂O₃/rGO, respectively. The irreversible peak of 1.57 V is attributed to the lithium intercalation into Fe₂O₃ fiber: Fe₂O₃ + xLi⁺ + xe⁻ → Li_xFe₂O₃, disappearing at subsequent cycles. The cathodic peak of 0.90 V was ascribed to the phenomenon that hexagonal Li_xFe₂O₃ is transformed to cubic Li₂Fe₂O₃: Li_xFe₂O₃ + (2 − x)Li⁺ + (2 − x)e⁻ → Li₂Fe₂O₃. The strongest peak of (0.60–0.75 V) corresponds to the stepwise reduction of Fe³⁺ to Fe²⁺ and Fe⁰ and the formation of the SEI layer:⁴⁰ Li₂Fe₂O₃ + 4Li⁺ + 4e⁻ ↔ 2Fe⁰ + 3Li₂O. The disappearance of the strongest peak in the subsequent cycle was due to the irreversible reactions induced by the formation of an SEI layer on the electrode in the first cycle. At the same time, the peaks at 1.57 V and 0.90 V disappeared indicating that lithium insertion and phase transformation from Li_xFe₂O₃ to Li₂Fe₂O₃ are irreversible.⁴¹ On the other hand, the two anodic peaks at 1.64 and 1.86 V in the first scan corresponded to the oxidation process of Fe⁰ to Fe²⁺ and Fe³⁺.⁴² The broad peak appear in the subsequent cycles and their intensities remains almost the same, as observed from anodic curves, indicating enhanced stability during lithiation and delithiation processes.

Fig. 7 (a) CV curves of Fe₂O₃ (I)/rGO composites at 0.1 mV s⁻¹; (b) Initial discharge–charge curves of Fe₂O₃ (I)/rGO composites at 0.1 A g⁻¹; (c) second-cycle discharge and charge curves of the Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO composites at 0.1 A g⁻¹. (d) Rate performance of Fe₂O₃ (I) fiber, Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO composites; (e) long term cycling performance and coulombic efficiency of Fe₂O₃ (I)/rGO composites at 5 A g⁻¹; (f) electrochemical impedance spectroscopy of the different electrodes.

Fig. 7b shows the charge–discharge curves of the Fe₂O₃ (I)/rGO composites anode at various cycles, which were tested at a current rate of 100 mA g⁻¹ within a voltage range of 0.02–3.0 V. It is found that the Fe₂O₃ (I)/rGO composite electrode exhibits discharge and charge plateaus at ∼0.9–0.8 V and ∼1.5–1.9 V that are typical for Fe₂O₃ based materials, which is in agreement with the charge–discharge process as shown in Fig. 7a. It is also observed that the initial charge and discharge capacities are around 1095.3 and 1303.7 mA h g⁻¹, and the initial coulombic efficiencies are 84%. In the second and third cycles, although the discharge capacities decrease gradually to about 1098.8 and 1081.6 mA h g⁻¹, the coulombic efficiencies are all above 97%. Furthermore, the typical second discharge–charge curves of the Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO composites at a current rate of 100 mA g⁻¹ within a voltage range of 0.02–3.0 V are shown in Fig. 7c. The discharge capacity of the Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO composite was 1093.6, 1062.3 and 578.7 mA h g⁻¹, respectively. From the comparison, the Fe₂O₃ (I)/rGO with a smallest diameter show obvious improved specific capacity and higher Fe₂O₃ utilization.

To compare the nobility of Fe₂O₃/rGO composites of electrochemical characteristics, the rate capabilities of Fe₂O₃/rGO composites of Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO and Fe₂O₃ fibers as anodes were examined at the various current densities of 0.1, 0.2, 0.5, 2 and 0.1 A g⁻¹ (Fig. 7d). The Fe₂O₃ (III)/rGO electrode exhibited the lowest rate capability for three samples, significantly decreasing from 610.5 mA h g⁻¹ at 0.1 A g⁻¹ to 287.9 A h g⁻¹ at 2 A g⁻¹. By contrast, the Fe₂O₃ (I)/rGO electrode exhibited a more stable cyclic performance than others at high current rates and delivered capacities of 1085.2, 1065.1, 974.5, 866.1, and 1170.9 mA h g⁻¹ at 0.1, 0.2, 0.5, 2 and 0.1 A g⁻¹, respectively.

Then, the cyclic stability of the Fe₂O₃ (I)/rGO was investigated as shown in Fig. 7e. It were cycled under an high current density of 5 A g⁻¹, and the Fe₂O₃/rGO composites show high stability with a specific capacity of 407.8 mA h g⁻¹ after 1500 cycles compared to the Fe₂O₃ at the same test conditions and the excellent columbic efficiency of higher than 99%. Most notably, the cyclability and rate performance of Fe₂O₃/rGO are better than most of the lately reported Fe₂O₃-based anode materials, such as porous Fe₂O₃ nanotubes,¹¹ nanoropes,¹³ hollow nanobarrels/rGO¹⁴ and hollow nanoparticles/rGO.¹⁵

In order to understand the fast ion transport in the composite electrodes, electrochemical impedance spectroscopy (EIS) measurements were conducted on the fresh cell with Fe₂O₃ (I) fiber, Fe₂O₃ (I)/rGO, Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO composite electrodes (Fig. 7f). The Nyquist plot shows a depressed semicircle, which generally describes two components: charge transfer resistance⁴³ and interfacial resistance.⁴⁴ The straight line in the low frequency region is the Warburg constant, which is assigned to the diffusion and transport of the Li⁺ ion from the electrolyte to the surface of the electrode. The electrochemical system can be simply modeled by a Randles equivalent circuit, where R_s is the ohmic resistance, C_d is the double-layer capacitance, R_ct is the charge transfer resistance, and Z_w is the Warburg impedance describing the solid-state diffusion of Li⁺ in Fe₂O₃.⁴⁵ The resistance of the depressed semicircle of Fe₂O₃ (I)/rGO is lower than that of Fe₂O₃ (II)/rGO and Fe₂O₃ (III)/rGO, which is 78, 118 and 163 Ω, respectively. It suggests that the Fe₂O₃ (I)/rGO composite have a much lower electrolyte resistance and charge transfer resistance than other samples. These results confirm that such special structure for the Fe₂O₃ (I)/rGO composites with Fe₂O₃ fiber as the “corncob” and multilayer reduced graphene oxide as the “sepal” is beneficial to further reduce the electronic conductivity of composites. Also, the low impedance can be attributed to higher surface area, uniform Fe₂O₃ nanofiber distribution in graphene layer, and better electrolyte wettability associated with porous structure of the Fe₂O₃/rGO composite material.

Finally, to further understand the excellent rate and long cycle performance of the Fe₂O₃/rGO composite electrodes based on above results, the structural and morphological evolution of the electrodes were measured by SEM and TEM, respectively in different cycle stages: 100th cycle, 500th cycle and 1000th cycle. As shown in Fig. 8(a and d) and (b and e), it is found that the film electrodes have maintained the fiber morphology before 500 cycles though some Fe₂O₃/rGO composite fiber have fractured and some fiber diameters get thin during the discharge–charge process from 100 to 500 cycles. At the 1000^th cycle, the nanowires are intermixed with each other and begun to get together (Fig. 8c and f), but these Fe₂O₃ nanoparticles are still homogeneously distributed on the graphene sheets, which maintains the reversible the high specific capacity of the electrode material. In the meantime, the electrochemical impedance spectroscopy (EIS) measurements after initial, 500, and 1000 cycles have also confirmed the stability of Fe₂O₃/rGO composite electrode (Fig. S7†). The charge transfer resistance of the Fe₂O₃ (I)/rGO composite anode are 78, 101 and 195 Ω, respectively. It suggests that the Fe₂O₃ (I)/rGO composite have low charge transfer resistances, though the values become larger with the cycle number for initial to 1000 cycles gradually. The low charge transfer can be derived from the Fe₂O₃ nanofibers or nanoparticles encapsulated in the high conductive graphenen network.^46,47 In short, the aforementioned factors of as-prepared Fe₂O₃ fiber/rGO composites translate to greater accessibility to active sites for the lithium ions, shorter diffusion distances and quicker lithium ion diffusion, thereby explaining the significantly better rate capability and excellent cycling performance of the Fe₂O₃ fiber/rGO composite electrodes.

Fig. 8 The SEM (a–c) and TEM (d–f) images of Fe₂O₃/rGO composites after different cycles: (a and d), 100 cycles; (b and e), 500 cycles; (c and f), 1000 cycles.

4. Conclusions

In summary, we have designed and fabricated a “corn” structure for a scalable and electrochemically stable Fe₂O₃/rGO electrode. The fabrication process is carried out by an electrospinning technique assisted with anneal treatment and far infrared reduction process. The “corn” structure consists of Fe₂O₃ fibers completely protected by multilayer rGO. Natural void space between the Fe₂O₃ fibers and rGO allows for the expansion of Fe₂O₃; and the good surface area and unblocked channels in the Fe₂O₃ fiber facilitates fast diffusion of Li⁺. Interestingly, this electrode shows excellent capacity (1085.2 mA h g⁻¹ at 0.1 A g⁻¹), long cycle life (407.8 mA h g⁻¹ at 5 A g⁻¹ for 1500 cycles), and high coulombic efficiency (99%). In addition to Fe₂O₃, this “corn” structure can also be applied to other high capacity anode materials for next generation Li-ion batteries to improve cycle life and coulombic efficiency.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financial supported by the National Natural Science Foundation of China (No. 21263016, 21363015, 51662029) and the Jiangxi Province Research Program of Science and Technology (No. 2011BBE50022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra08846a

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