Facile synthesis of Fe₂O₃/MWCNT composites with improved cycling stability

Abstract

In this study, Fe₂O₃/multi-walled carbon nanotube (Fe₂O₃/MWCNT) composites were synthesized via vacuum solution absorption and subsequent calcination treatment. The amount of Fe₂O₃ and MWCNT components, crystalline structure, morphology and electrochemical performance of the as-prepared material were characterized by thermo-gravimetric (TG) analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) method, energy dispersive spectrometry (EDS) and charge–discharge tests. Results show that Fe₂O₃ nano-particles, with a diameter of about 23 nm, loaded in the void space in the intertwined MWCNT matrix or on the MWCNT homogeneously. The as-obtained Fe₂O₃/MWCNT composites have a relatively small BET surface area, pore size and pore volume compared to that of pure MWCNT. Electrochemical measurements show that the Fe₂O₃/MWCNT composites exhibit a high reversible capacity of 1026 mA h g⁻¹ after 50 cycles at a charge–discharge rate of 0.2 C. The improved performance may be ascribed to the nano-sized Fe₂O₃ with a faster Li⁺ diffusion coefficient which can release the volume expansion effectively. On the other hand, the MWCNT can act as a buffering matrix and electron conductors in the composites.

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1. Introduction

In recent years, the increasing demands on portable, high performance energy storage devices have excited the interest of investigators of lithium ion batteries.^1–3 In order to improve the performance, new electrode materials with high specific capacities are necessary. Recently, transition metal oxides (M_xO_y, M = Fe, Mn, Ni, Cu, etc.)^2,4–11 with remarkably high specific capacities have attracted much attention¹² and have been researched as the next generation of promising anode materials for high energy-density lithium ion batteries. Their theoretical specific capacity is above 600 mA h g⁻¹ (for example, 1005 mA h g⁻¹ for Fe₂O₃, 718 mA h g⁻¹ for NiO^13,14), which is much higher than that of the conventional graphite anode material (372 mA h g⁻¹).⁸ Although these transition metal oxides are so attractive, their commercial use is still hindered. The most critical problem is their large volume change during charge–discharge process, which causes rapid anode disintegration under the induced mechanical stress, and the rapid capacity fading during cycling.^15,16 One effective way to resolve this problem is coating the oxides with carbon or forming composites with carbon materials, such as nano-carbon, graphene, carbon nanotubes and so on.^17–20 Among these composites, the carbon materials can act as a barrier to suppress the aggregation of active particles and thus increase their structural stability during cycling, and also act as a buffering matrix to relax the expansion that occurred within the electrode upon lithiation/delithiation process.

The common synthesis methods to prepare transition metal oxide/carbon composites are sol–gel, hydrothermal synthesis and chemical precipitation method etc.^4,19,21,22 Although these methods offer some advantages, defects are obvious too. For example, expensive metal alkoxide, complex process and high-level equipment are needed, which presents bottlenecks with respect to large-scale manufacture. At present, the mass production of high performance electrode materials has yet to be realized.

Vacuum solution absorption method is a process that taking in the metal salt solution to the porous carbon material under vacuum, which can assure more sufficient impregnation. This method has been used in the preparation of SnO₂, et al.²³ There are few reports on the synthesis of Fe₂O₃/MWCNT composites via this method. In this work, a vacuum solution absorption method was used to synthesize Fe₂O₃/multi-walled carbon nanotubes (MWCNT) composites. And its morphology, structure and electrochemical performance of the Fe₂O₃/MWCNT composites were characterized.

2. Experimental

2.1. Materials

Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). MWCNT 20–30 nm in diameter and over 10 μm in length, whose carbon content is 95%, were purchased from Chengdu Organic Chemicals Co. Ltd, Chinese Academy of Sciences. All other reagents were of analytical grade and were used as received without any purification process. Water used was distilled.

2.2. Synthesis of Fe₂O₃/MWCNT composites

A certain amount of Fe(NO₃)₃·9H₂O as precursor was dissolved in distilled water under vigorous stirring to form a homogenous saturated precursor aqueous solution. MWCNT, which were dried at 120 °C in vacuum for 12 h, were added to a single necked bottle. The weight ratio of MWCNT to the final product Fe₂O₃ calcined from Fe(NO₃)₃·9H₂O is 1 : 3. Subsequently, vacuumed the bottle to a reduced pressure (−0.15 MPa) under constant magnetic stirring for 20 min at 60 °C with water bath. Then Fe(NO₃)₃ aqueous solution was added into the vacuumed bottle, which will be absorbed by the MWCNT immediately. Then the MWCNT were put into an open glass beaker to evaporate the distilled water at 60 °C for 12 h. Finally, the dried material was subjected to calcination at 300 °C for 2 h in a high purity nitrogen atmosphere to form the final product, Fe₂O₃/MWCNT composites.

2.3. Material characterization

Crystal structure of the Fe₂O₃/MWCNT composites was characterized by X-ray diffraction (XRD, Bruker AXS D8) with Cu Kα radiation. Thermo-gravimetric (TG) analysis of the Fe₂O₃/MWCNT composites was investigated by thermal analysis apparatus (TG, STA449C, German Netzsch). Particle morphology was conducted with a scanning electron microscope (SEM, 7800F, JEOL) and a transmission electron microscope (TEM, JEM-2100, JEOL). Elements analysis was carried out with a scanning electron microscope equipped with an energy dispersive spectroscope (EDS, Q70, Bruker Nano GmbH). Specific surface area and porosity analysis for the composites were performed through Brunauer–Emmett–Teller (BET, Tristar 3020, Micromeritics) nitrogen adsorption–desorption measurements. Galvanostatic charge–discharge experiment data were collected using LAND Cell test system (CT2001A, Wuhan).

2.4. Electrochemical measurements

The electrochemical performance was evaluated using CR2016 coin cell. The working electrode was prepared by casting slurries consisting of the as-prepared powder, Super P carbon black as conductive agent and polyvinylidene fluoride (PVDF) as the binder in a weight ratio of 80 : 10 : 10 on a copper foil with N-methyl-2-pyrrolidine (NMP) as solvent. Subsequently, the electrode was dried at 80 °C for 12 h under the vacuum. The separator was a polypropylene membrane (Celgard 2400). Lithium metal was used as counter and reference electrode. The electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 volume). The cells were discharge–charged in the voltage range of 0.01–3.0 V (vs. Li/Li⁺). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were conducted with an electrochemical workstation (SI1260, Solartron) using fresh cells at open potential. A.C. impedance spectra were measured at 10 mV amplitude with a frequency range from 0.1 Hz to 100 kHz. The CV measurement was performed in the range of 0.01–3 V at a scanning rate of 0.1 mV s⁻¹. Moreover, after 50 cycles, the A.C. impedance spectra were measured again at the same condition.

3. Results and discussion

3.1. Crystal structure of the Fe₂O₃/MWCNT composites

Fig. 1 illustrates the XRD pattern of the as prepared Fe₂O₃/MWCNT composites. The broad hump at about 25°–27° (2θ), as the star indicated in the figure, corresponds to the turbostratic structure of MWCNT in the composites.²⁴ All the other diffraction peaks are well indexed to the rhombohedral phase of hematite (JCPDS no. 33-0664), indicating the formation of single-phase spinel α-Fe₂O₃. The intensity of the characteristic peaks is strong, indicating a high crystallinity. The crystalline size calculated from Scherrer equation is about 23 nm, proving that a nano crystallized Fe₂O₃ was formed.

Fig. 1 XRD pattern of the Fe₂O₃/MWCNT composites.

3.2. TG analysis

Thermo-gravimetric (TG) curve of the Fe₂O₃/MWCNT composites measured from 30 °C to 750 °C in air are displayed in Fig. 2. The heating rate is 10 °C min⁻¹. And the air flow rate is 50 mL min⁻¹. It can be noticed that a slight weight loss about 1.5% occurs before 200 °C, which corresponding to the evaporation of the adsorbed water molecules in the Fe₂O₃/MWCNT composites. The huge weight loss about 25% occurring between 380 °C to 600 °C can be attributed to the oxidation of the carbon nanotubes in the final composites. Therefore, the results indicate that the mass percentage of Fe₂O₃ is about 75% of the Fe₂O₃/MWCNT composites, which is close to the pre-set proportion.

Fig. 2 TG pattern of the Fe₂O₃/MWCNT composites.

3.3. The morphology and composition of the Fe₂O₃/MWCNT composites

Elemental composition and morphology of the as-prepared Fe₂O₃/MWCNT composites were confirmed with EDS, SEM and TEM. In Fig. 3, Fe, C and O peaks are observed, suggesting that the samples are mainly composed of C, Fe and O. The atom ratio of C/Fe/O is ∼46 : 20 : 34 for the composites, which agrees with the value calculated from the TG analysis and the formula of hematite. Fig. 4(a) and (b) illustrate the images of the final synthesized composites. One can see that the Fe₂O₃ particles locate in the void space in the intertwined MWCNT matrix or on the MWCNT, while not in the MWCNT, which might because the diameter of the MWCNT used is too small. Fig. 4(c) show the high resolution TEM (HRTEM) images of the square area in Fig. 4(b). The stripes width of the crystalline domain in the composite are 0.35 nm and 0.25 nm, corresponding to the (002) plane of MWCNT and (110) plane of Fe₂O₃, respectively. It is supposed that nano Fe₂O₃ particles can decrease the path length for Li ion transport, release the volume expansion effectively. Moreover, MWCNT in Fe₂O₃/MWCNT composites can act as a buffering matrix to relax the expansion of the electrode active material during the lithiation/delithiation process and act as a barrier to suppress the aggregation of active particles. On the other hand, MWCNT can effectively increase the electrical conductivity of the Fe₂O₃ particles. So the electrochemical performance and cycling stability will be enhanced.

Fig. 3 EDS pattern of the Fe₂O₃/MWCNT. The inset is the element content table of Fe₂O₃/MWCNT.

Fig. 4 (a) SEM and (b) TEM images of the Fe₂O₃/MWCNT composites, (c) HRTEM images of the Fe₂O₃/MWCNT composites.

Brunauer–Emmett–Teller (BET) measurements were performed on pure MWCNT in contrast to the Fe₂O₃/MWCNT composites. As shown in Fig. 5 and Table 1, both the two nitrogen adsorption–desorption isotherms can be attributed to type IV with a distinct hysteresis loop observed in the range of 0.5–1.0 P/P₀. The as-obtained Fe₂O₃/MWCNT composites have a relatively smaller BET surface area, pore size and pore volume than that of pure MWCNT, which is primarily due to the formation of Fe₂O₃ in and on the carbon nanotubes.

Fig. 5 Nitrogen adsorption–desorption isotherms at 77 K of the pure MWCNT and the Fe₂O₃/MWCNT composites.

Table 1 BET results for the pure MWCNT and the Fe₂O₃/MWCNT composites

Sample	Surface area (m^2 g^−1)	Pore size (nm)	Pore volume (cm^3 g^−1)
MWCNT	109	196	0.539
The Fe2O3/MWCNT composites	103	126	0.328

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3.4. Electrochemical performance

CV curves of the Fe₂O₃/MWCNT electrode are shown in Fig. 6. In the cathodic process of the first cycle, an obvious peak located at about 0.5 V was observed, which can be attributed to the reduction of Fe³⁺ to Fe⁰ and to some irreversible reaction of the electrolyte. In the anodic process, a broad peak located at about 1.8 V was observed, which corresponds to the oxidative reactions of Fe⁰ to Fe³⁺.^25–27 In the 50th cycle, the cathodic and anodic peaks shift to higher potential. The displacement of anodic peak is smaller than that of the cathodic peak, indicating a good reversibility of the Fe⁰ to Fe³⁺ reaction. In addition, the decreasing peak intensity indicates the decreasing capacity.

Fig. 6 CV plots of the Fe₂O₃/MWCNT composites.

Fig. 7 shows the charge–discharge and cycling performance of the Fe₂O₃/MWCNT composites and pure MWCNT. Fig. 7(a) shows the initial two and the 50th cycles charge–discharge voltage profiles of the Fe₂O₃/MWCNT electrode at a current density of 0.2 C rate. The voltage range is 0.01–3.0 V. In the first discharge curve, it exhibits an declined plot between 1.70 V and 0.80 V and an extended voltage plateau at 0.80 V, which are ascribed to the conversion from Fe³⁺ to Fe⁰. And a more declined discharge slope between 0.80 V and 0.01 V originates from the partial capacity contribution from the MWCNT. In the charge curves, the sloped region from 1.30 V to 2.35 V corresponds to the reversible oxidation of Fe⁰ to Fe³⁺.^17,21,28

Fig. 7 (a and c) The discharge–charge curves of the Fe₂O₃/MWCNT composites and pure MWCNT in the initial two and the 50th cycles; (b and d) cycling performance of the Fe₂O₃/MWCNT composites and pure MWCNT at a charge–discharge rate of 0.2 C.

The Fe₂O₃/MWCNT composites delivers an initial discharge capacity of 1264 mA h g⁻¹ and charge capacity of 1201 mA h g⁻¹. One can find that the stable capacity of pure MWCNT used is about 260 mA h g⁻¹. Considering the Fe₂O₃/MWCNT composite electrode composition, its specific capacity is much higher than the theoretical capacity. This phenomenon has been widely documented in the literatures for transition metal oxides,^13,29,30 which can be ascribed to the reversible formation and decomposition of a polymeric gel-like film on the surface of the electrode particles. The initial coulombic efficiency of the electrode is as high as 95%, which is much higher than that of transition metal oxides in most other reports,^7,16,31 inferring the significantly decreased irreversible capacity of the composite electrode. This is believed to be resulted from the decreased formation of the solid–electrolyte interface (SEI), which should be attributed to the unique hybrid structure of the Fe₂O₃/MWCNT composite. From the second cycle, the discharge capacity decreases gradually from 1208 mA h g⁻¹ to 1026 mA h g⁻¹ (the 50th cycle), with a capacity retention of 81.2%, which is much higher than that of the Fe₂O₃/MWCNT composites synthesized via hydrothermal method.³² It is supposed that nano-sized Fe₂O₃ is benefit for diffusion of Li⁺, and can release the volume expansion effectively. Moreover, MWCNT in Fe₂O₃/MWCNT composites can act as a buffering matrix to relax the expansion of the electrode active material during the lithiation/delithiation process and act as a barrier to suppress the aggregation of active particles. On the other hand, MWCNT can effectively increase the electrical conductivity of the Fe₂O₃ particles. Thus the electrode structural stability is increased.

The impedance spectra of the composite electrode before and after cycling are shown in Fig. 8. The Nyquist plots in Fig. 8 display one semicircles and one straight line, which describes the charge transfer resistance (R_ct) and Warburg impedance, respectively. Before cycling, the electrode demonstrated small semicircle, indicating low resistances of the charge-transfer reaction. After 50 cycles, the semicircle was enlarged, suggesting increases in R_ct. This is probably due to the large volume change during charge–discharge cycling that gradually deteriorates the contact between the active material particles and the conductive matrix.

Fig. 8 The impedance spectra of the Fe₂O₃/MWCNT composites before and after 50 cycle.

4. Conclusions

Fe₂O₃/multi-walled carbon nanotube (Fe₂O₃/MWCNT) composites have been successfully synthesized via vacuum solution absorption and a subsequent calcination treatment. Electrochemical measurements show that MWCNT played an important role in improving the performance of cycle stability. The Fe₂O₃/MWCNT electrodes exhibit a high reversible capacity of 1026 mA h g⁻¹ at a current density of 0.2 C at the 50th cycle. This high performance can be ascribed to the MWCNT which act as a buffering matrix and electron conductors in the composites, and on the other hand, the nano-sized Fe₂O₃ with faster Li⁺ diffusion coefficient can release the volume expansion effectively.

Acknowledgements

The work was supported by the Major Science and Technology Projects in Henan province (121100210500).

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