Gelatin assisted wet chemistry synthesis of high quality β-FeOOH nanorods anchored on graphene nanosheets with superior lithium-ion battery application

Abstract

High quality β-FeOOH nanorods anchored on graphene nanosheets to form β-FeOOH/rGO hybrid nanostructures were synthesized by using a gelatin assisted wet chemistry strategy. Owing to their unique structures, the as-harvested β-FeOOH/rGO hybrids were used as anode electrodes with superior electrochemical performance for lithium ion batteries.

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Introduction

Lithium-ion batteries (LIBs) provide the dominant power source for a wide range of devices, such as portable electronic devices and electromobiles.¹ They are considered to be one of the promising candidates for solving the rapid depletion of fossil fuels.² As important components of LIBs, anode materials play an important role in affecting the LIB performance.³ However, traditional carbonaceous-materials based anode materials can only support a low Li storage capacity of 372 mA h g⁻¹ and relatively low lithium diffusion rates, which cannot meet the ever increasing needs of the high performance LIBs.⁴

Recently, transitional metal oxides (TMOs) with high Li storage capacity and fast recharging rates have shown great potential as alternative anode materials for LIBs.⁵ Specifically, the TMOs–carbonaceous hybrids have started to be explored as effective and promising anode materials⁶ because of their enhanced electrical conductivity⁷ and capability to release the mechanical stress induced by the lithium insertion and extraction process.⁸ Amongst the TMOs–carbonaceous materials, TMOs–graphene hybrid anode materials have drawn particular research interest for LIBs⁹ owing to the graphene possessing excellent electrical conductivity, mechanical stability, as well as large surface areas and so on.¹⁰

The intrinsic properties of tunnel structured iron oxyhydroxides (β-FeOOH), such as it has the band gap of ∼2.12 eV, relatively low cost, and high capacity, make it as a promising anode materials for LIBs.¹¹ However, up to now, few reports focused on the synthesis of high quality β-FeOOH nanostructures and improvement of their corresponding LIBs performances. In this paper, for the first time, we synthesized high quality β-FeOOH nanorods anchored on reduced graphene oxide (rGO) nanosheets (β-FeOOH/rGO hybrid nanostructures) through a gelatin assisted one-step wet chemistry method. The as-harvested products were used as anode materials for LIBs (Scheme 1), which exhibited electrochemical performance with high capacities of 290.43 and 870.50 mA h g⁻¹ at a charge–discharge rate of 0.10 A g⁻¹ for pure β-FeOOH nanorods and β-FeOOH/rGO hybrid nanostructures, respectively. This demonstrates rGO as an efficient intermediate to facilitate the migration of electron and lithium ions.

Scheme 1 Schematic illustration of the formation β-FeOOH/rGO hybrid nanostructures and their application for lithium-ion battery.

Results and discussion

The as-formed β-FeOOH/rGO hybrid nanostructures are systematically characterized by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) techniques. The TEM image shown in Fig. 1a demonstrates that the β-FeOOH nanorods are well anchored on the graphene nanosheets. The size distribution histogram of β-FeOOH nanorods is depicted in Fig. S1.† For comparison, the TEM image of pure rGO nanostructures is shown in Fig. S2.† A digital photo (inset of Fig. 1a) demonstrates the as-obtained β-FeOOH/rGO hybrid nanostructures are readily dispersed and highly stable in absolute ethanol. Fig. 1b shows the HRTEM image taken from a typical β-FeOOH/rGO hybrid nanostructure in Fig. 1a, and it clearly illustrates the β-FeOOH nanorods grew onto the rGO. The clear-cut crystal lattice fringes with interplanar spacing of ∼0.12 nm corresponded to the (211) plane of tetragonal phase β-FeOOH. Meanwhile, the SEM and HAADF-STEM images presented in Fig. 1c and d demonstrate the β-FeOOH nanorods are supported on rGO surface with high morphological yield, and the flexible graphene nanosheets could act as a special 2D structure framework to form the as-observed hybrid nanostructures.

Fig. 1 (a) TEM, (b) HRTEM, (c) SEM and (d) HADDF-STEM images of β-FeOOH/rGO hybrid nanostructures. Inset of (a): a digital photograph of colloidal β-FeOOH/rGO hybrid nanostructures dispersed in absolute ethanol.

Fig. 2a shows the powder X-ray diffraction (XRD) patterns of the as-synthesized β-FeOOH nanorods and β-FeOOH/rGO hybrid nanostructures, and all of the diffraction peaks are assigned to the pure tetragonal phase of β-FeOOH (space group: I4/m with lattice constants: a = 10.535 Å, b = 10.535 Å, c = 3.030 Å, JCPDS: 34-1266). No diffraction peaks from any other chemical species are detectable. The chemical states of Fe and O elements in the β-FeOOH/rGO hybrid nanostructures are examined by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 2b–d. In Fig. 2b, peaks assignable to core levels of Fe 2p, O 1s, C 1s, N 1s are identified,¹² and the double peaks located at 711.0 and 724.7 eV are ascribed to the core levels of Fe 2p_3/2 and Fe 2p_1/2 of β-FeOOH, respectively. Meanwhile, two shakeup peaks at 718.9 eV and 733.4 eV can also be observed, these results indicate that the chemical state of Fe ions for the β-FeOOH/rGO hybrid nanostructures are mainly trivalent.¹³ The two intense peaks located at 530.0 and 531.6 eV (Fig. 2d) are attributed to the core levels of crystal lattice oxygen and chemisorbed oxygen, respectively.¹⁴

Fig. 2 (a) XRD patterns of β-FeOOH nanorods and β-FeOOH/rGO hybrid nanostructures; (b) XPS survey spectra, (c) Fe 2p and (d) O 1s XPS signals of β-FeOOH/rGO hybrid nanostructures.

Fourier transform infrared (FTIR) spectroscopy of the β-FeOOH nanorods and β-FeOOH/rGO hybrid nanostructures are recorded as shown in Fig. S3a.† The characteristic bands located at 1630, 1380, and 890 cm⁻¹ are assigned to the Fe–O vibrational modes in β-FeOOH.¹⁵ The Raman spectrum is shown in Fig. S3b.† Two different types of bands located at 1354 cm⁻¹ and 1595 cm⁻¹ are detected, which could be assigned to the D-band and G-band of rGO. In general, G-band is due to the first order scattering of E_2g mode and D-band is the disorder of bond angle and edge defects.¹⁶ In addition, the peaks located at 208, 281, 310, 390 and 720 cm⁻¹ further indicate the trivalent Fe exists in β-FeOOH/rGO hybrid nanostructures.^11d,17

To investigate the lithium storage behaviour of pure β-FeOOH nanorods and β-FeOOH/rGO hybrid nanostructures, the electrochemical properties have been investigated based on typical coin half cells, where the metallic lithium serves as the counter electrode. Fig. 3a depicts the representative cyclic voltammograms (CV) curves of β-FeOOH/rGO hybrid nanostructures at a scan rate of 0.50 mV s⁻¹ in the voltage range of 0.00–3.00 V. The corresponding CV curves of pure β-FeOOH are shown in Fig. S4a† for comparison (scan rate: 0.50 mV s⁻¹; voltage range: 0.00–3.00 V). In the first cathodic scanning, two cathodic peaks centred at 1.65 V, 0.60 V are detected respectively. Meanwhile, the two broad peaks at 1.70 V and 0.25 V can be found in the anodic scanning. The cathodic peaks are ascribed to the reaction between Li and FeOOH producing the Fe nanoparticles embedded in an amorphous matrix of LiOH and Li₂O.¹⁸ The first cathodic peak at ∼1.65 V is associated with the transition from the FeOOH to Li_xFeOOH (x < 1).^9a After that, the second cathodic peak at ∼0.60 V, can be ascribed to the formation of Li_xFeOOH (x < 2)^9a and the valence of iron at this stage is likely to be Fe(0),^9b,c while in the subsequent anodic scan, besides the reaction peak of β-FeOOH at 1.70 V, another anodic peak located at 0.25 V appears, and this is ascribed to the synergistic reaction between β-FeOOH and rGO.¹⁹ Starting from the second cycle, there is only a slight migration of the main reduction peak (from 0.60 V to 0.70 V); this phenomenon may possibly result from the irreversible phase transformation with the formation of the solid electrolyte interface (SEI) layer in the first cycle.^20a Meanwhile, the other anodic peaks remain unchanged, indicating good reversible reaction of Fe(0) to Fe(III). In the subsequent process, no obvious changes are observed for the anodic and cathodic peaks, implying the good reversibility and structure stability during charge and discharge process. Several aspects may account for such phenomena: (1) rGO nanosheets could act as an intermediary for electronic/ionic transfer between β-FeOOH nanorods; (2) the large surface area of β-FeOOH/rGO (∼88.48 m² g⁻¹, Fig. S5†) triggers the interfacial storage ability, especially in low potentials which also contributes to the capacity;^20b (3) the formation of SEI layers. The mechanisms for lithium storage could be elucidated by the following reactions:²¹

Fe(III)OOH + Li⁺ + e⁻ ↔ LiFe(II)OOH (partially reversible) (1)

LiFe(II)OOH + xLi⁺ + xe⁻ ↔ Li_1+xFe(II − x)OOH (partially reversible) (2)

Li_1+xFe(II − x)OOH + (2 − x) Li⁺ + (2 − x) e⁻ ↔ Fe(0) + Li₂O + LiOH (highly reversible) (3)

Formation and deformation of SEI layers and interfacial storage. (4) Fig. 3b depicts the charge–discharge profiles of β-FeOOH/rGO hybrid nanostructures for the first, second, and tenth cycles at a current density of 0.10 A g⁻¹ within 0.00–3.00 V and the corresponding charge–discharge profiles of pure β-FeOOH are shown in Fig. S4b† (current density: 0.10 A g⁻¹, 0.00–3.00 V). As seen from Fig. 3b, two potential plateaus (∼1.65 V and ∼0.60 V) are observable in the first discharge, which is matched well with the above CV curves (Fig. 3a). A high discharge capacity of 870.50 mA h g⁻¹ and the corresponding charge capacity of 807.50 mA h g⁻¹ are delivered in the 1st Li intercalation process, leading to a coulombic efficiency of 92.76%. This large discharge capacity is attributed to the electrolyte decomposition and subsequent formation of the SEI layer.²² The first charge curve exhibits a sloped region at ∼1.65 V and it can be contributed to the oxidation reaction of iron. During the 2nd cycle, the electrode shows a discharge capacity of 783.80 mA h g⁻¹ and the corresponding charge capacity of 781.50 mA h g⁻¹, leading to a much higher coulombic efficiency of 99.70%. For comparison, charge–discharge profiles of pure β-FeOOH nanorods were tested under the same condition (Fig. S4b†). The first discharge and charge capacities for pure β-FeOOH anode were 290.43 mA h g⁻¹ and 243.21 mA h g⁻¹, with coulombic efficiency of 83.73%. However, serious capacity decay was observed with cycling and the capacity of the pure β-FeOOH anode decreased to 160.37 mA h g⁻¹ after 10 cycles.

Fig. 3 Electrochemical measurements of β-FeOOH/rGO hybrid nanostructures: (a) CV between 0.00 V and 3.00 V at a scan rate of 0.50 mV s⁻¹; (b) charge–discharge profiles at a current density of 0.10 A g⁻¹; (c) cycling performance at a current density of 0.10 A g⁻¹; (d) rate ability at different current density.

The cycling performance of half cells of pure β-FeOOH and β-FeOOH/rGO hybrid nanostructures half cells are evaluated by galvanostatic measurement at current density of 0.10 A g⁻¹. As shown in Fig. 3c, a little decay about 86.7 mA h g⁻¹ can be observed at the second cycle. The discharge capacity of β-FeOOH/rGO hybrid nanostructures electrode is 870.5 mA h g⁻¹ and 783.8 mA h g⁻¹ for the first and the second cycles, and the reduction value is ∼86.7 mA h g⁻¹. This decrease may be due to the generation of SEI layer on the composites surface.²³ However, there is no obviously decreased at the subsequent cycles. In contrast, large capacity decay is observed for pure β-FeOOH nanorods electrode from the first cycle to 100th cycles at same current density (Fig. S4c†). The reasons for higher reversible capacity of β-FeOOH/rGO hybrid nanostructures electrode can be attributed to the β-FeOOH suited on rGO nanosheets since rGO nanosheets may serve as an elastic and highly conductive framework to maintain the electrical contact between the β-FeOOH nanorods and the current collectors.²⁴

Rate capabilities of the pure β-FeOOH nanorods and β-FeOOH/rGO hybrid nanostructures under various current densities from 0.10 to 1.00 A g⁻¹ in steps and then returned to 0.10 A g⁻¹. As shown in Fig. 3d, the β-FeOOH/rGO hybrid nanostructures electrode exhibits final discharge capacities of 757.22, 520.41, 449.62, 366.53, 319.91 and 266.72 mA h g⁻¹ at current densities of 0.10, 0.20, 0.40, 0.60, 0.80 and 1.00 A g⁻¹, respectively. It should be noted that this reversibility performance was much better than the pure β-FeOOH nanorods (Fig. S4d†). After 60 cycles of charge and discharge at various current densities, the discharge capacity of the β-FeOOH/rGO hybrid nanostructures electrode can still recover to 681.21 mA h g⁻¹ at 0.10 A g⁻¹, indicating the outstanding rate performance and resilience of the electrode. In contrast, pure β-FeOOH nanorods electrode exhibited relatively poor capacities and rate capabilities (Fig. S4d†) and it gives only 11.64 mA h g⁻¹ at 1.00 A g⁻¹ and cannot recover its initial level when the current density returns to 0.10 A g⁻¹. This rate capabilities improvement may be due to the rGO, since rGO nanosheets could facilitate the electronic/ionic migration between the composites and also could buffer the volume expansions that cause the pulverization of the active material. In summary, when β-FeOOH nanorods anchored on the rGO, the LIBs performance in terms of capacity, cycling stability and rate capacity, was enhanced obviously compared to the pure β-FeOOH.

To understand the reason for excellent cycling performance of as-obtained β-FeOOH/rGO hybrid nanostructures, the morphology and phase of β-FeOOH/rGO electrode after cycling are investigated in detail. Fig. S6† shows the representative TEM image of the β-FeOOH/rGO hybrid nanostructures after 100 discharge/charge cycles at 0.10 A g⁻¹. Since rGO nanosheets played the important role in the growth of high quality β-FeOOH nanorods, it apparently endowed the benefit to enlarge the interface reaction area between the electrode and electrolyte. On account of the flexible rGO nanosheets, the volume expansions could also be effectively accommodated, leading to the highly reversible stability. Therefore, the synergetic effect of the high surface area of β-FeOOH nanorods and conductive rGO nanosheets may contribute to higher electrochemical performances of the as-prepared β-FeOOH/rGO hybrid nanostructures compared to the pure β-FeOOH nanorods.

Conclusions

In summary, high quality β-FeOOH nanorods were synthesized by a facile gelatin assisted one-step wet chemistry method and these β-FeOOH nanorods were anchored on rGO nanosheets to improve the electrochemical performance as anode materials for LIBs application. Compared to the pure β-FeOOH nanorods, the β-FeOOH/rGO hybrid nanostructures electrode delivered a higher discharge capacity of 650.22 mA h g⁻¹ after 100 cycles at 0.10 A g⁻¹. The improved LIBs performances were attributed to the presence of rGO, which not only facilitated the electronic/ionic migration between the composites but also buffered the volume expansions, which caused the capacity fading of the active material.

Experimental section

Materials

Ferric chloride hydrates (FeCl₃·6H₂O, 99.50%, Tianjin Zhiyuan Chemical Company), gelatin (C₁₀₂H₁₅₁N₃₁O₃₉, G7041-500G, 99.00%, Sigma-Aldrich), urea (CO(NH₂)₂, 99.90%, Sigma-Aldrich), graphene oxide (GO, 99.00%, Suzhou Hengqiu Scientific and Technical Corporation) and absolute ethanol (C₂H₆O, >99.70%, Guangdong Guanghua Scientific and Technical Corporation) were used as received without further purification.

Synthesis of β-FeOOH nanorods

In a typical procedure, firstly, 27.02 g (100 mmol) of FeCl₃·6H₂O and 1.00 g of gelatin were added into a 100 mL round-bottom flask with vigorous magnetic stirring to form transparent solution at room temperature for ∼1 h. After that, 6.06 g (100 mmol) of urea was dissolved in the FeCl₃/gelatin mixture stirring at room temperature for ∼30 min, and the resultant homogeneous solution was then transferred into a 100 mL Teflon-lined autoclave. The autoclave was elevated at 80 °C for 21 h. After cooled down to room temperature, the as-formed β-FeOOH were collected by centrifugation at 8000 rpm for 5 min and washed with absolute ethanol for three times, and then dried in a vacuum at 60 °C overnight.

Synthesis of β-FeOOH/rGO hybrid nanostructures

The synthetic procedure was similar to that of β-FeOOH nanorods, except that after 27.02 g (100 mmol) of FeCl₃·6H₂O, 1.00 g of gelatin and 6.06 g (100 mmol) of urea formed transparent solution, 200 mg of graphene oxides (GO) was dissolved in the FeCl₃/gelatin/urea mixture stirring at room temperature for ∼30 min, then the resultant homogeneous solution was transferred into a 200 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 80 °C for 21 h. After cooled down to room temperature, the as-formed β-FeOOH/rGO were collected by centrifugation at 8000 rpm for 5 min and washed with absolute ethanol for three times, and then dried in a vacuum at 60 °C overnight.

Characterization

Powder X-ray diffraction (PXRD) patterns of the products were recorded on Rigaku D/MAX-RB (Japan) at a scanning rate of 5° min⁻¹ from 10° to 70°, using Cu Kα radiation (λ = 1.5406 Å). Transmission electron microscope (TEM) analysis was performed with a Hitachi HT-7700 (Japan) transmission electron microscope operating at 100 kV. Scanning electron microscopy (SEM) analysis was performed with a Carl Zeiss Sigma (Germanic) operated at 20 kV. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) characterizations were performed with a FeiTecnat G2 F20S-Twin (USA) operated at 200 kV. The Fourier transforms infrared absorption (FTIR) spectra of the products were carried on NICOLET 6700 FT-IR (USA). X-ray photoelectron spectroscopy (XPS) spectra were obtained using an Escalab 250 xi photoelectron spectrometer using Al Kα radiation (15 kV, 225 W, base pressure ≈ 5 × 10⁻¹⁰ Torr). Raman spectra of powder samples were recorded on LabRAM HR Raman microscope with a laser excitation wavelength of 532 nm.

Electrochemical measurements

For the electrode preparation, 80 wt% of the as-prepared β-FeOOH and β-FeOOH/rGO samples, 10 wt% acetylene black (Super-P), and 10 wt% polyvinylidene fluoride (PVDF) binder were mixed with N-methyl-2-pyrrolidinone (NMP). The obtained slurry was coated onto Cu foil disks to form working electrodes, which were then dried in a vacuum at 100 °C for 12 h to remove the solvent. Electrochemical measurements were carried out on CR2032 (3.00 V) coin type cells with lithium metal as the counter/reference electrode, a celgard 2500 membrane as the separator, and electrolyte solution obtained by dissolving 1 M LiPF₆ into a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC–DMC, 1 : 1, v/v). The coin cells were assembled in an Ar-filled glove box with concentrations of moisture and oxygen below 1.0 ppm. The charge–discharge tests were performed with a LANHE battery tester at a voltage window of 0.00–3.00 V for β-FeOOH/rGO. Cyclic voltammetry (0.00–3.00 V, 0.50 mV s⁻¹) was performed with an electrochemical workstation (CHI 660C).

Acknowledgements

We gratefully acknowledge the financial aid from the start-up funding from Xi'an Jiaotong University, the Fundamental Research Funds for the Central Universities (2015qngz12), the China National Funds for Excellent Young Scientists (grant no. 21522106) and NSFC (grant no. 21371140). The authors thank Prof. Shujiang Ding from Xi'an Jiaotong University of China for the useful discussions for electrochemical data. We also thank Dr Xinghua Li from Northwest University (China) for the HRTEM characterization.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Length and diameter distribution histogram, TEM image of pure graphene oxides (GO), FTIR and Raman spectra of β-FeOOH/rGO, electrochemical performance data of pure β-FeOOH, TEM image of β-FeOOH/rGO after 100 cycles. See DOI: 10.1039/c5ra28170a

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