Enhanced rate capability of a lithium ion battery anode based on liquid–solid-solution assembly of Fe₂O₃ on crumpled graphene

Abstract

We report a liquid–solid-solution assemble strategy to fabricate Fe₂O₃@graphene (Fe₂O₃@rGO) composites at the oil/water interface, where the Fe₂O₃ nanoplates with thickness of about 100 nm are anchored on crumpled graphene sheets. The in situ nucleation and growth process can ensure intimate contact between Fe₂O₃ nanoplates and graphene sheets, while the oil shell on Fe₂O₃ can prevent the aggregation of the Fe₂O₃@rGO composite. The crumpled structure and the relatively thin Fe₂O₃ nanoplates can shorten the electron diffusion path and enhanced the utilization rate of the active material. When used as the anode material, the Fe₂O₃@rGO anode shows a high reversible capacity of 1160 mA h g⁻¹ at 0.2 A g⁻¹ after 100 cycles, as well as a high cycling stability (101.3% capacity retention after 300 cycles at 1 A g⁻¹). Moreover, with ∼156 s charging time (at a current density of 12.8 A g⁻¹), the anode can deliver a significant capacity of 552 mA h g⁻¹, indicating its promising application as a high-rate lithium ion battery anode.

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Introduction

Lithium ion batteries (LIBs) with high energy density, long lifespan and low cost are one of the most promising candidates for next-generation energy storage devices.^1–3 Si,^4,5 Co₃O₄,^6,7 Fe₂O₃,^8,9 NiO^10,11 etc. with high theoretical specific capacity have been widely investigated for high performance LIB anodes. Among them, Fe₂O₃ with high theoretical capacity (∼1005 mA h g⁻¹), low cost, and being abundant and environmental benign in nature, has become a promising candidate for advanced electrodes. However, the conductivity of the Fe₂O₃ monolith is limited, which lead to a poor rate performance at high current density. Meanwhile, the large volume expansion of Fe₂O₃ anodes during cycling can result in serious pulverization, which influence the irreversible capacity and cycle life.¹² Hence, development of Fe₂O₃-based anode materials with high capacity, rate capability and cycling stability is desired for next-generation electrodes.

Various strategies have proposed to avoid the pulverization and thereby enhance the structural stability and rate performance of the Fe₂O₃. One strategy is to construct nanosized architecture, such as nanorods,¹³ nanofibers,¹⁴ hollow-structures,¹⁵ nanoplates,¹⁶ which can shorten the diffusion paths of Li⁺ and electrons, and release the strain caused by the volume change. Another one is to hybridize the nanosized active materials with conductive materials to enhance the conductivity of the composites, such as activated carbon,¹⁷ carbon nanotube,¹⁸ graphene,^19–21 conductive polymer²² et al. Among them, graphene with high conductivity, two dimensional structure and good mechanical property, has become the first choice to fabricate advanced electrodes. Graphene sheets in the electrodes can not only facilitate the electron transfer to the active materials, the superior mechanical property and flexibility can also buffer the volume change of host material during discharge/charge process. Though the study of Fe₂O₃/graphene hybrids has made great progress,^9,23 anodes with high rate performance as well as high stability is still urgently demanded.

Here, we report a liquid–solid-solution assemble strategy to fabricate Fe₂O₃@graphene (short for Fe₂O₃@rGO) composite, where the Fe₂O₃ nanoplates with average diameter of about 500 nm in width and 100 nm in thickness are anchored on graphene sheets. The crumpled structure and the relatively thin Fe₂O₃ nanoplates can shorten the electron diffusion path and enhanced the utilization rate of the active material, while the intimate contact can facilitate the electron transfer from the conductive graphene sheets to Fe₂O₃. With such a structure, the Fe₂O₃@rGO anode shows a high reversible capacity of 1160 mA h g⁻¹ at 0.2 A g⁻¹ after 100 cycles, as well as a high cycling stability (101.3% capacity retention after 300 cycles at 1 A g⁻¹). With ∼156 s charging time (at a current density of 12.8 A g⁻¹), the anode can deliver a significant capacity of 552 mA h g⁻¹, which is comparable to most of the state-of-the-art anodes, indicating its promising application as a high-rate LIB anode.

Experimental methods

Synthesis of Fe₂O₃@rGO composite

Graphene oxide (GO) was synthesized from graphite by a modified Hummers' method. Fe₂O₃@rGO composite (sample 7 : 3) was fabricated through a liquid–solid-solution (LSS) process. Briefly, (NH₄)₂Fe(SO₄)₂·6H₂O (0.24 g) was dissolved in GO aqueous solution (∼1 mg mL⁻¹, 20 mL) to form a Fe²⁺ solution under ultrasonic (solution A, Fe₂O₃/GO experimental designed weight ratio of 7 : 3). Then, NaOH (1.50 g), DI water (10 mL), ethanol (20 mL) and oleic acid (10 mL) were mixed orderly under vigorous stir to form a clear solution (solution B). After addition of solution A dropwise to solution B under vigorous stir, the color of the mixture changed from black-brown to black. After stir for another 30 min, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, and treated at 180 °C for 10 h. After cooling to room temperature naturally, the black precipitates were separated by centrifugation, washed with ethanol and DI water for several times, respectively. The product was obtained after frozen drying and thermal treatment at 500 °C for 4 h under N₂ atmosphere. For comparison, other composites (Fe₂O₃/GO experimental designed weight ratio of 5 : 5 and 9 : 1) were prepared using the same process with different Fe₂O₃/GO ratio as the contrast samples.

Structure characterization

X-ray powder diffraction (XRD) patterns were obtained by an X'Pert Pro MPD using a Cu Kα radiation. The morphology and structure were characterized by field-emission scanning microscope (SEM, JSM-5900LV) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G20). The actual content of the samples were determined by Q500 Thermogravimetric analyzer (TA instruments, USA) with a heating rate of 10 °C min⁻¹ under air flow. X-ray photoelectron spectroscopy (XPS) analysis was performed using a PHI5000 Versa Probe. Raman measurements were carried out by HORIBA Jobin Yvon LabRAM HR with a 532 nm Nd-Yag laser.

Electrochemical characterization

Electrochemical experiments were performed using CR2032 coin-type cells. The working electrodes were prepared by mixing active material, acetylene black and poly(vinylidene fluoride) (8 : 1 : 1 in weight) in N-methyl pyrrolidone to form a slurry, then coated onto a copper disc and dried under vacuum at 120 °C for 12 h. The mass loading of each electrodes (including active materials and additives) was ∼1 mg cm⁻². Pure Li foil and polypropylene membrane (Celgard 2400) was used as the counter electrode and the separator, respectively, and 1 M LiPF₆ in a mixture of ethylene carbonate and diethyl carbonate (1 : 1 by volume) was used as the electrolyte. Cell assembly was carried out in an Ar protected glovebox. The galvanostatic charge–discharge measurements (0.01–3 V) were performed on a LANHE (CT2001A) battery test system. Cyclic voltammetry (CV) was performed on a LK9805 electrochemical analyser under a scan rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was carried out on a Zahner electrochemical workstation in a frequency range of 0.01 Hz to 100 kHz with an amplitude of 10 mV. The specific capacity is calculated based upon the total active materials (Fe₂O₃ and rGO).

Results and discussion

The Fe₂O₃@rGO composite is fabricated through a novel liquid–solid-solution (LSS) process, where Fe₂O₃ nanoplates are formed at the oil/water (O/W) microemulsion interface on graphene (Fig. 1).^24–27 Firstly, (NH₄)₂Fe(SO₄)₂ is dissolved in GO aqueous suspension, where Fe²⁺ ions can anchor on the oxygen-containing groups of GO sheets via electrostatic adsorption. Ethanol can act as the mutual solvent to bridge the water and oleic acid to form microemulsion with liquid phase (ethanol–oleic acid) and solution phase (ethanol–water). After that, NaOH can react with oleic acid to form sodium oleate, which has a hydrophilic head (sodium salt side) and a hydrophobic chain (carbon chain). After mixing these two solutions, an ion exchange process occurs between Na⁺ and Fe²⁺, and then ferrous oleate/GO complex is obtained. The hydrophobic chain of the ferrous oleates on GO sheets together with sodium oleates form a liquid (oil) phase shell outside the graphene sheets (the hydrophobic chain form the oil shell while the hydrophilic head is distributed on both side of the oil shell).²⁴ During hydrothermal treatment, the Fe²⁺ on GO sheets can act as the nucleus and grow on the GO sheets, while the phase transfer process can ensure plenty of Fe²⁺ ions delivered from aqueous solution to solid phase (GO sheet interface) by oleate. The in situ nucleation and growth process in LSS process can ensure intimate contact between Fe₂O₃ nanoplates and graphene sheets, while the oil shell on Fe₂O₃ nanoplates can prevent the aggregation of the Fe₂O₃@rGO composite. To the best of our knowledge, this LSS assembly process is the first time reported to fabricate the metal oxide/graphene composites.

Fig. 1 Illustration of the formation mechanism of Fe₂O₃@rGO composite.

The obtained composites before and after heat treatment are characterized by X-ray diffraction (XRD) to trace the crystal transition. Fig. 2a shows all peaks of products are in good agreement with α-Fe₂O₃ (PDF #33-1346), α-FeOOH (PDF #29-2713) and γ-Fe₂O₃ (PDF #39-1346). This result reveals that the as-prepared composite is composed of hematite (α-Fe₂O₃) and goethite (α-FeOOH), while they are converted to γ-Fe₂O₃ during heat treatment via a dissolution–crystallization and dehydration–rearrangement process, respectively.²⁸ Furthermore, since the XRD patterns of Fe₃O₄ (PDF #19-0629) and the γ-Fe₂O₃ are similar, X-ray photoelectron spectroscopy (XPS) measurement has been carried out to verify the Fe²⁺ and Fe³⁺ cations (Fig. 2b). The full spectrum indicates the presence of carbon, oxygen, and iron element, while the inset Fe 2p spectrum reveals two broad peaks and a satellite peak of Fe³⁺ around 710.9, 724.3 and 719.8 eV, respectively, all of which indicates the formation of γ-Fe₂O₃.¹⁵ Thermogravimetric analysis (TGA) of Fe₂O₃@rGO (Fig. 2c) reveals that the content of Fe₂O₃ in the composites are 70.9%, 81.4% and 92.9%, respectively (corresponds to Fe₂O₃@rGO (5 : 5), Fe₂O₃@rGO (7 : 3) and Fe₂O₃@rGO (9 : 1)), indicates the gradually increase of initial GO content can increase the mass content of rGO in the composites. The difference between the actual content and the experimentally designed value may be due to the deoxidization of GO sheets and the incomplete transfer of Fe ions. The Raman spectrums of Fe₂O₃@rGO (Fig. 2d) reveals the G bands of the Fe₂O₃@rGO (5 : 5), Fe₂O₃@rGO (7 : 3) composite shift from 1575 cm⁻¹ to 1588 cm⁻¹ and 1593 cm⁻¹, respectively. The blue shift of the G band may introduced by the interaction between the graphene sheets and Fe₂O₃ nanoplates.²⁹

Fig. 2 (a) XRD patterns of Fe₂O₃/α-FeOOH@rGO and γ-Fe₂O₃@rGO. (b) XPS spectrum of Fe₂O₃@rGO, inset is Fe 2p XPS spectrum of Fe₂O₃@rGO. (c) TGA curves and (d) Raman spectrums of Fe₂O₃@rGO with different Fe₂O₃ content.

Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) are taken to investigate the morphology and structure evolution of the Fe₂O₃@rGO composites. Fig. 3a and b present the structure of the composite before heat treatment (Fe₂O₃/FeOOH@rGO). The composite reveals a highly crumpled graphene sheet, and the Fe₂O₃/FeOOH nanoplates are uniformly anchored on graphene sheets without apparent aggregation, which may benefit by oil film protection during the LSS assembly. After heat treatment, the morphology of the obtained Fe₂O₃@rGO (7 : 3) composite does not change significantly (Fig. 3c and d).

Fig. 3 SEM images of (a, b) Fe₂O₃/α-FeOOH@rGO (7 : 3) and (c, d) Fe₂O₃@rGO (7 : 3) at different magnifications, and (e, f) TEM images of Fe₂O₃@rGO (7 : 3) at different magnifications.

Fe₂O₃ nanoplates uniformly anchored on crumpled graphene sheets are observed. The Fe₂O₃ nanoplates are about 500 nm in width and 100 nm in thickness (Fig. 3e and f). Such thin nanoplates can shorten the electron diffusion path and enhanced the utilization rate of the active material. Furthermore, the composite structure can be retained after sonication, indicating good interaction between the graphene sheets and Fe₂O₃ nanoplates. On the contrary, the Fe₂O₃@rGO (5 : 5) composites show similar structure but with thicker graphene sheets, while the Fe₂O₃@rGO (9 : 1) shows highly aggregation of the Fe₂O₃ nanoplates on graphene sheets (Fig. S1, see ESI†). Moreover, the size of the Fe₂O₃ nanoplates show decrease trend with the decrease of the graphene content in the composites. This may be due to the different graphene content. The nucleus can be formed both on GO surface and in solution with insufficient GO sheets, which results in smaller Fe₂O₃ crystal size. The Fe₂O₃ nanoparticles suspended in solution trend to aggregation with Fe₂O₃@graphene composite to reduce the surface energy. In general, this crumpled Fe₂O₃@rGO composite may act as a high performance anode: the crumpled structure and the relatively thin Fe₂O₃ nanoplates can shorten the electrolyte ions diffusion pathway inside the electrode, while the intimate contact can facilitate the electron transfer rate from the conductive graphene sheets to active material (Fe₂O₃). Furthermore, the graphene sheets with high mechanical property can also buffer the stress from the volume expansion of the Fe₂O₃ nanoplates during cycling.

The electrochemical measurements of Fe₂O₃@rGO composites are firstly evaluated by cyclic voltammetry (CV). Fig. 4a shows the CV curves of Fe₂O₃@rGO (7 : 3) electrode for the initial five cycles. During the first discharge cycle, the minor peaks at 1.58 and 1.18 V correspond to the initial lithiation reaction to form Li_xFe₂O₃.¹⁶ The shoulder peak at 0.90 V is related to the phase change from hexagonal Li_xFe₂O₃ to cubic Li₂Fe₂O₃ for further Li-insertion.¹⁶ The sharp peak at 0.66 V is caused by the reduction of Fe(III) to Fe(0) as well as the formation of solid electrolyte interface (SEI). During the charge cycle, the anodic peaks at 1.62 and 1.81 V are associated with the gradual oxidation of Fe(0) to FeO and then to Fe₂O₃.¹² In the following cycles, the center and the strength of the peaks remain the same, which reveals a high reversibility of the composite. This reversibility and stability can also be observed by the galvanostatic charge/discharge test. Fig. 4b exhibits the charge/discharge curves of Fe₂O₃@rGO (7 : 3) at current density of 0.2 A g⁻¹. After the first cycle, the charge/discharge curves of the 5th, 10th and 50th cycle show similar plateau voltage, as well as the specific capacity, indicating high stability and reversibility of the Fe₂O₃@rGO (7 : 3) electrodes.

Fig. 4 (a) CV curves and (b) galvanostatic charge/discharge profiles of Fe₂O₃@rGO (7 : 3) electrode at a scan rate of 0.1 mV s⁻¹ and a current density of 0.2 A g⁻¹, respectively. (c) Cycling performance of Fe₂O₃@rGO electrodes at a current density of 0.2 A g⁻¹, and (d) rate performance of Fe₂O₃@rGO electrodes at different current densities with various graphene amount. (e) Cycling performance of Fe₂O₃@rGO (7 : 3) at current density of 1, 6.4 and 12.8 A g⁻¹, respectively. (f) Specific capacity at different current densities of the Fe₂O₃@rGO (7 : 3) electrode, comparing with some state-of-the-art LIBs anodes.

The cycling and rate performance of the Fe₂O₃@rGO (7 : 3) electrodes are investigated. As shown in Fig. 4c, an initial reversible specific capacity of 969 mA h g⁻¹ is obtained, which is comparable to most of the iron-based electrodes.^14,30–33 After the slight decrease in the subsequent several cycles, the reversible specific capacity of Fe₂O₃@rGO (7 : 3) increases gradually, and reaches a high value of 1160 mA h g⁻¹ after 100 cycles. This increase tendency and higher than theoretical specific capacity may be attributed to the gradual activation of Fe₂O₃, the synergistic effect of graphene and Fe₂O₃, as well as the pseudo-capacitive and the electron double layer capacitance (EDLC) of oxygen-containing graphene sheets.^16,20,30 Furthermore, the Fe₂O₃@rGO (7 : 3) electrodes exhibit a high rate performance, the specific capacity decreases slightly with the increase of current density (Fig. 4d). The discharge capacities are 994.7, 950.3, 883.4, 837.2, 763.2 and 655.2 mA h g⁻¹ at the current densities of 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 A g⁻¹, respectively. When the current density increases to as high as 12.8 A g⁻¹ (corresponds to a charge time of 156 s), the specific capacity still reaches a high value of 552 mA h g⁻¹, indicating its potential as a high rate anode. Moreover, the reversible capacity can recover and keep the increase tendency once the current density is restored to 0.2 A g⁻¹, implying good reversibility of the anode.

In order to evaluate the cyclic stability under different charge rate, the Fe₂O₃@rGO (7 : 3) electrode tested under different current densities are also evaluated. The electrode show good long-term cycling stability (Fig. 4e), at a current density of 1 A g⁻¹, the anode maintains a high capacity of 838.2 mA h g⁻¹ (101.3% capacity retention) after 300 cycles with a coulombic efficiency of almost 100%. When the current densities are up to 6.4 and 12.8 A g⁻¹, the discharge capacities are 636.4 and 419.8 mA h g⁻¹ after 400 cycles and with a capacity retention of the about 107% and 81%, respectively, indicates the stable structure and high-rate performance of the Fe₂O₃@rGO 7 : 3 composite.

For comparison, the electrochemical performance of the Fe₂O₃@rGO (5 : 5) and Fe₂O₃@rGO (9 : 1) electrodes are also investigated (Fig. 4c and d), both of them show inferior cycling and rate performance than the Fe₂O₃@rGO (7 : 3) electrode. The Fe₂O₃@rGO (5 : 5) electrode shows lower specific capacity value in cycling test and quick decrease during rate performance. The lower content of the active materials (Fe₂O₃), as well as the relatively larger Fe₂O₃ particle size and thicker graphene layer of the Fe₂O₃@rGO (5 : 5) electrode may hinder the electrolyte ions diffusion inside the electrode. On the other hand, the Fe₂O₃@rGO (9 : 1) composite show a high specific capacity in the beginning, then the values drop quickly in both the cycling and rate performance test. This may due to the lack of contact between graphene sheets and aggregated Fe₂O₃ nanoparticles, which can cause a high electron transfer resistance. To further understand the superior electrochemical performance of the Fe₂O₃@rGO (7 : 3) electrode, electrochemical impedance spectroscopy (EIS) measurements of these anodes are performed after 20 cycles activation. The Nyquist plots (Fig. S2, see ESI†) display that the diameter of the semicircle for Fe₂O₃@rGO (7 : 3) electrode in the high frequency region is smaller than the other two anodes, indicating lower ions and electrons diffusion resistance.³⁴ This result indicates that Fe₂O₃@rGO (7 : 3) electrode possesses the most balanced performance between the electron transfer and the electrolyte ions diffusion resistance, and explains its best electrochemical performance.

Besides the electrodes with different graphene content, some latest literature works on iron-based high-rate LIBs anodes are also taken into comparison. As shown in Fig. 4f, the rate performance of the Fe₂O₃@rGO (7 : 3) electrode is comparable or higher than most of the state-of-the-art electrodes (including carbon-based anodes, and other metal oxide based anodes), especially under high current densities.^{8,9,13–16,35–42} This improved rate performance of the Fe₂O₃@rGO (7 : 3) anode is attributed to its well designed structure: the electrical conductive graphene network and the good interaction between Fe₂O₃ and rGO ensures good electrical contacts and small charge transfer resistance; meanwhile, the graphene sheets can effectively relieve the stress of volume expansion upon material conversion process, reduces the pulverization and agglomeration of the electrode material. Considering the simple and scalable preparation method and low cost, this composite shows promise for application as high rate LIB anodes.

Conclusions

In summary, we fabricate Fe₂O₃ nanoplates anchored graphene composite with crumpled structure via a liquid–solid-solution assemble strategy. The crumpled structure as well as intimate contact between Fe₂O₃ nanoplates and graphene can facilitate the ions and electrons transfer inside the composite. The Fe₂O₃@rGO composites as anode shows a high reversible capacity of 1160 mA h g⁻¹ at 0.2 A g⁻¹ after 100 cycles, as well as a high cycling stability (101.3% capacity retention after 300 cycles at 1 A g⁻¹). With ∼156 s charging time, the anode can deliver a significant capacity of 552 mA h g⁻¹, which is comparable to most of the high-end iron-based anodes, indicating its promising application as a high-rate LIB anode.

Acknowledgements

We appreciate the National Natural Science Foundation of China (21506130, 21490582).

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Footnote

† Electronic supplementary information (ESI) available: Additional characterization data of SEM images, electrochemical impedance spectroscopy (EIS) measurements are presented. See DOI: 10.1039/c5ra22408j

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