Self-assembled graphene-constructed hollow Fe₂O₃ spheres with controllable size for high lithium storage

Abstract

Graphene-constructed hollow Fe₂O₃ spheres (GHFs) were prepared by a one-pot hydrothermal process. The Fe₂O₃ particles were perfectly constructed using graphene sheets. This strategy was an easy method for the large-scale synthesis of GHFs. The size of Fe₂O₃ spheres ranged from 2000 nm to 50 nm and can be easily controlled by changing the weight ratio of GO to FeCl₂, and the size greater than 250 nm shows a hollow structure obviously. As the anode material for lithium-ion batteries, the GHFs (300 nm) showed an excellent reversible capacity of 950 mA h g⁻¹ after 50 cycles at a charge–discharge rate of 100 mA g⁻¹, and delivered a reversible capacity as high as 640 mA h g⁻¹ at a high rate of 1000 mA g⁻¹. The outstanding electrochemical performance of GHFs can be attributed to the graphene-constructed hollow Fe₂O₃ spheres and the synergistic interaction between uniformly dispersed Fe₂O₃ particles and graphene. Moreover, the favorable performance of GHFs can be attributed to the reduced diffusion length of lithium, in which the hollow structure of Fe₂O₃ spheres played an important role.

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1. Introduction

Lithium-ion batteries (LIBs) are some of the most promising types of batteries because of their high energy density, low maintenance, and relatively low self-discharge.^1–3 In classical commercial LIBs, graphitic carbon is the most commonly used anode material. Development of new electrode materials with high energy densities has been one of the most important pursuits to satisfy the ever-growing demand for high performance LIBs.^4–6 Nanostructured metal oxides, (MOs) such as SnO₂,^7,8 TiO₂,⁹ Co₃O₄,^10,11 MnO₂,^12–14 Mn₃O₄,¹⁵ Fe₃O₄,¹⁶ and Fe₂O₃,¹⁷ are regarded as potential anode materials for LIBs because of their high reversible capacity, high power capability, safety, and long cycle life. Among them, Fe₂O₃ has attracted considerable attention, owing to its high theoretical specific capacity (1005 mA h g⁻¹), low cost, and is environmentally-safe.^18–20 However, low conductivity and pulverization problem, which can cause a breakdown in electrical contact pathways between MO particles, lead to rapid capacity fading during charge–discharge cycling.^17,21,22 To address these problems, conducting carbon matrices were used to buffer volume changes and improve structural stability of electrodes.^23–25

Graphene, a honeycomb network of sp² carbon lattices, has been considered as one of the most appealing carbon matrices for MO particles because of outstanding charge carrier mobility and mechanical robustness.^26,27 Nevertheless, to the best of our knowledge, Fe₂O₃ usually grows on the surface of graphene in other works,^21,28,29 and hollow structure is usually fabricated with template,^30–32 a study on graphene-constructed hollow Fe₂O₃ spheres with size-controlled synthesis has not been reported up to date. Therefore, developing size-controlled graphene-constructed G/MO hybrids that can address aggregation of nanoparticles is highly desirable.

In this work, a novel class of GHFs was fabricated by a one-pot hydrothermal method and freeze-drying process. Fe₂O₃ particles, with uniform dispersion and similar sizes, were synthesized by a hydrothermal procedure using FeCl₂ and graphene oxide (GO) as precursors. The overall synthesis procedure of GHFs is illustrated in Scheme 1. Compared with the Fe₂O₃ particles supported on graphene sheets, GHFs are perfectly constructed with uniform dispersion. Furthermore, GHFs were size-controlled and provided highly conductive networks with increased surface areas and short diffusion path lengths for lithium ion transport. As a result, GHFs exhibited outstanding reversible capacity and excellent rate performance (950 mA h g⁻¹ after 50 cycles at a charge–discharge rate of 100 mA g⁻¹ and reversible capacity 640 mA h g⁻¹ at a high rate of 1000 mA g⁻¹), when used as the anode material for lithium storage.

Scheme 1 Schematic of the synthesis route to GHFs.

2. Experimental section

2.1. Materials

Graphite flakes, NaNO₃, KMnO₄, 98% H₂SO₄, 30% H₂O₂, FeCl₂·4H₂O, 37% HCl, iron powder, and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals, except FeCl₂·4H₂O, were of analytical grade. FeCl₂ solution, 0.2 M (freshly-prepared).

2.2. Synthesis of HCHFs

GO was synthesized from natural graphite flakes using a modified Hummers method. Exfoliation was carried out by ultrasonication of the GO dispersion under ambient conditions. FeCl₂ (0.2 g ml⁻¹) was synthesized from FeCl₂·4H₂O as follows. FeCl₂·4H₂O (15.7 g) was dissolved in distilled water to a volume of 100 cm³. For the preparation of GHFs, 60 ml of 2.5 mg ml⁻¹ GO suspension was added to 30 g ethanol, and ultrasonicated for 10 min. FeCl₂ (0.5 ml 0.2 g ml⁻¹) was added to 10 g ethanol. GO was added fast to the FeCl₂–ethanol mixture, then 3 ml H₂O₂ (wt% = 30) was added slowly with vigorous stirring, and then ultrasonicated for 10 min at room temperature. The resulting suspension was sealed in a 200 ml Teflon-lined autoclave and hydrothermally treated at 180 °C for 10 h. The prepared sample was on dialysis for 7 days, and freeze-dried overnight. For comparison purposes, different volumes of FeCl₂, ranging from 0.2 ml to 3 ml, were used. Bare Fe₂O₃ particles were also synthesized without GO addition.

2.3. Characterization of materials

Morphology of the samples was investigated with a field-emission scanning electron microscopy (FESEM) system (FEI, Sirion 200). Materials were characterized by power X-ray diffraction (XRD) using a Rigaku X-ray diffractometer with Cu-Kα irradiation (λ = 0.15406 nm) at 40 kV, 20 mA over the 2θ range from 10° to 70°. Thermogravimetric analysis (TGA) was employed by a TA Q5000IR with a heating rate of 10 °C min⁻¹ under flowing air. Nitrogen adsorption/desorption isotherms at 77 K were determined by Micrometrics SAP 2010.

2.4. Electrochemical measurements

Electrochemical properties of the samples were evaluated with CR 2010 coin cells. Test electrodes were prepared by mixing active materials with conductive carbon black (super P) as the conductive agent and polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP) as the binder in a weight ratio of 80 : 10 : 10 to form a slurry, which was then coated onto a copper foil. The mass of active material on each anode was 1.0 mg. Pure lithium foil was used as counter electrodes. Celgard 2400 microporous polypropylene membrane was used as a separator. The electrolyte consisted of a solution of 1 M LiPF₆ in ethylene carbonate–dimethyl carbonate–diethyl carbonate (1 : 1 : 1 weight ratio). CR 2016 coin cells were assembled in an argon-filled glovebox with water and oxygen content less than 1 ppm. Discharge and charge measurement was carried out with a LAND 2001A system with cutoff potentials 0.01 V for discharge and 3.0 V for charge.

3. Results and discussion

The synthesis route to GHFs is illustrated in Scheme 1. First, Fe²⁺ cations from FeCl₂ bind with oxygen-containing groups on GO sheets through electrostatic interactions. Second, Fe(OH)₃ was freshly synthesized after addition of H₂O₂. In this process, Fe(OH)₃ and GO hydrogels were dispersed homogeneously. Third, Fe₂O₃ transformed from Fe(OH)₃ hydrogel was constructed by GO and self-assembled, forming a hollow sphere by hydrothermal treatment. GO is simultaneously transformed into RGO. Finally, dark grey GHFs are obtained after dialysis and freeze-drying.

The morphology and microstructure of the synthesized GHFs were elucidated by means of FESEM and nitrogen adsorption/desorption analysis. FESEM images of cross-sections of GHFs (Fig. 1) clearly show Fe₂O₃ particles (greater than 200 nm) with a hollow structure and almost all particles were perfectly covered with graphene. The encapsulated graphene sheet can efficiently prevent aggregation of particles and prevent direct contact between Fe₂O₃ particles and the electrolyte. Results reveal that graphene is a notably thin, well-defined, and interconnected network. Geometric confinement of MO particles within graphene layers were reported to enhance interface contact and suppress dissolution and agglomeration of particles, thereby promoting electrochemical activity and stability of the composites. Fe₂O₃ particles ranked randomly, so some hollow structure will be covered, more photos was attached in Fig. S4.†

Fig. 1 (a–c) Typical FESEM image of GHFs revealing graphene-constructed structure and Fe₂O₃ particle diameter of 2000, 300, and 100 nm. (d and e) Nitrogen adsorption/desorption isotherms and pore size distribution of Fe₂O₃/GAs.

A possible hypothesis of the hollow Fe₂O₃ sphere: first, in a typical process, GO should be salted out while FeCl₂ was added, but in this experiment, FeCl₃ and Fe(OH)₃ generated after H₂O₂ was added into the mixture under 0 °C, in this process, the water distributed in the mixture will be redistributed, and GO salted out will be redistributed into the water since Fe(OH)₃ should contain a large amount of H₂O, then a homogeneous solution with FeCl₃, Fe(OH)₃, GO, water and C₂H₅OH generated (Fig. S5†).

When the mixture was hydrothermally treated at 180 °C, Fe(OH)₃ close to GO loss the contained water fast and gather together as GO was a good thermal conductive material, in this process, graphene is also gather outside and greatly prevent the Fe(OH)₃ from gathering fast, until Fe₂O₃ was gather to generated a hard shell, Fe(OH)₃ covered inside will gather to the shell with graphene constructed, then, a hollow Fe₂O₃ sphere generated.

Brunauer–Emmmett–Teller analysis of nitrogen adsorption/desorption isotherms reveal that specific surface area of GHFs (Fe₂O₃ particle diameter of 300 nm) was 201 m² g⁻¹, which was much higher than that of bare Fe₂O₃. The pore volume was 0.244 cm³ g⁻¹ for GHFs. Moreover, the majority of pore sizes calculated by the Barret–Joyner–Halenda method are 1.6, 2.5, and 4 nm.

TGA measurement carried out in the air was used to determine the chemical composition of GHFs (Fe₂O₃ particle diameter of 300 nm). In Fig. 2a, the TGA curve displays a significant weight loss at approximately 450 °C and a constant weight above 500 °C. The minimal weight loss below 300 °C was probably caused by the evaporation of adsorbed water molecules. The major weight loss from 300 °C to 500 °C was approximately 20%, which indicates combustion of graphene. On the basis of calculations, Fe₂O₃ content of in GHFs was 78%. Crystalline structure of the final products was determined by XRD. XRD pattern of GHFs corresponds to the upper profile in Fig. 2b, and all the peaks can be attributed to Fe₂O₃ (JCPDS no. 33-0664). At 26°, an apparent diffraction peak corresponding to graphene was not observed in the XRD pattern of GHFs, which indicate the graphene in GHFs is dispersed uniformly without packed. The XRD pattern of the burned sample was pure Fe₂O₃ and date was attached in the Fig. S3,† and all the peaks can be attributed to Fe₂O₃ (JCPDS no. 33-0664).

Fig. 2 (a) TGA curves for Fe₂O₃ particles and GHFs in the air. (b) XRD patterns of GHFs and Fe₂O₃. (c) Cycling performance of GHFs (300 nm and 100 nm) and Fe₂O₃ at the current density of 100 mA g⁻¹. (d) Rate capacity of GHFs between 0.01 and 3.0 V with increasing current density. (e) Discharge/charge profiles of GHFs. Diameter of Fe₂O₃ particles in all GHFs is 300 nm.

Galvanostatic discharge (Li insertion)–charge (Li extraction) measurements were carried out at a current density of 100 mA g⁻¹ over a voltage range from 0.01–3.0 V to evaluate the electrochemical performance of the as-prepared GHFs. The first lithium insertion profile can be divided into three stages, namely, Fe₂O₃–Li_x, Fe₂O₃–cubic, and Li₂Fe₂O₃–Fe + Li₂O. At the early stage of lithium insertion (plateau I), a minimal amount of lithium was inserted into the crystalline structure of Fe₂O₃ before hexagonal to cubic stacking structural transformation of the close-packed anionic array. In the stage of lithium insertion (plateau II), a profile similar with plateau I was found, and a long plateau III appeared at approximately 0.8 V, corresponding to a reversible reaction between cubic Li₂Fe₂O₃ and Fe in the third stage.^33–35 The first discharge–charge step of GHFs (300 nm) delivered a specific discharge capacity of 1353 mA h g⁻¹ and charge capacity of 1120 mA h g⁻¹, with initial coulombic efficiency of 82.1%. The first discharge–charge step of GHFs (100 nm) delivered a specific discharge capacity of 1382 mA h g⁻¹ and charge capacity of 831 mA h g⁻¹, with initial coulombic efficiency of 60.1% and coulombic efficiencies were close to 98%. Typically, when the electrode size down to a certain point, the electrode pulverization can be effectively relieved, therefore facilitating the formation of a stable SEI. And the SEI formed in the lithiated expanded state can be broken as the nanostructure shrinks during delithiation.³⁶ In our materials, Fe₂O₃ was constructed by graphene, so the electrode pulverization can be effectively relieved, while the SEI could also be relieved by the stable structure and property of graphene. A direct comparison with Fe₂O₃ shows the synergistic effect between Fe₂O₃ and RGO. Fe₂O₃ without RGO covered delivered a specific discharge capacity of 1320 mA h g⁻¹, but a charge capacity of 710 mA h g⁻¹ (Fig. 2c). This initial capacity loss could be attributed to the formation of a solid electrolyte interphase (SEI) layer on the electrode surface during the first discharge step. At the end of 50 charge–discharge cycles, a reversible capacity as high as 950 mA h g⁻¹ of GHFs (300 nm) and 790 mA h g⁻¹ of GHFs (100 nm) can be retained, which was much higher than the theoretical specific capacity of graphene (372 mA h g⁻¹). The rate performances of GHFs at current rates of 100–1000 mA g⁻¹ are depicted in Fig. 2d. Reversible capacities were retained at 890 and 774 mA h g⁻¹ at 200 and 500 mA g⁻¹, and coulombic efficiencies were close to 98%. Remarkably, a reversible capacity of 640 mA h g⁻¹ can be delivered at a very high rate of 1000 mA g⁻¹. The prominent difference between GHFs and Fe₂O₃ emphasizes the efficiency of our protocol in the improvement of the electrochemical performance of Fe₂O₃ by incorporation with graphene. Moreover, performance stability of GHFs at high rates indicates ultrafast diffusion of lithium ions in bulk because of the short diffusion path length and stable graphene structure.

Thus, graphene architecture in GHFs not only improved the conductivity of the overall electrode, but also enhanced the electrochemical activity during the cycling process.

The high capacity, favorable cycling stability, and excellent rate capability of GHFs can be attributed to synergistic interactions between Fe₂O₃ particles and graphene associated with an interconnected macroporous framework. First, the graphene networks and the hollow Fe₂O₃ spheres provide a large surface area (201 m² g⁻¹) and efficiently reduce diffusion length for both electrons and lithium ions. Second, conductive graphene can serve as multidimensional pathways to facilitate transport of electrons in the bulk electrode. Finally, majority of Fe₂O₃ particles were encapsulated within the graphene sheets, which can suppress the aggregation of Fe₂O₃ particles to allow volume expansion during cycling.

4. Conclusions

In summary, monolithic GHFs were successfully fabricated by a one-pot hydrothermal reaction and subsequent freeze-drying process. Compared with the Fe₂O₃ particles supported on graphene sheets, GHFs are perfectly constructed with uniform dispersion. Furthermore, Fe₂O₃ particles were size-controlled and hollow. GHFs (300 nm) were applied as LIBs anodes and demonstrated superb enhancement of durability and rate performance with a very high reversible capacity of 950 mA h g⁻¹ at a rate of 100 mA g⁻¹, even after 50 cycles, 640 mA h g⁻¹ at a high rate of 1000 mA g⁻¹. Our present synthesis strategy could be further extended to the development of other graphene-based MO monoliths as high performance electrode materials with high specific capacities and rate capabilities in LIBs.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This project was supported by the Innovation Program of Shanghai Municipal Education Commission (Project number 09YZ387), Innovation Program of Shanghai Municipal Education Commission (Project number 11ZZ179), Science and Technology Commission of Shanghai Municipality (Project number 09QT1400600), ShuGuang Project (Project number 11SG54), National Natural Science Foundation of China (Project number 20976105) and Shanghai Leading Academic Discipline Project (Project number J51503), Shanghai Committee of Science and Technology, China (Project number 14520503200), the National Natural Science Foundation of China (Project number 41171250).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13270j

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