Preparation of rGO-wrapped magnetite nanocomposites and their energy storage properties

Abstract

Porous reduced graphene oxide (rGO)/Fe₃O₄ nanocomposites have been prepared in this study by a simple precipitation reaction followed by freeze drying. When the sample was dried at 80 °C, spinel phase of Fe₃O₄ wrapped by rGO sheets was obtained without any detectable impurities. It exhibits a high surface area of 30 m² g⁻¹. The rGO/Fe₃O₄ composites were also annealed under different conditions such as 600 °C in Ar, 700 °C in Ar and 700 °C in Ar–H₂ to understand the effect of annealing temperature on the electrochemical properties. The rGO content in the samples was found to be ∼3–4% in all the composites from CHNS analysis. Electrochemical properties of the different composites were investigated by cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy studies. The composite prepared at 80 °C exhibits very high capacity of 1254 mA h g⁻¹ for the first charge cycle. However, the capacity faded to 1046 mA h g⁻¹ at the end of 40 cycles. Though the high temperature annealed samples show slightly decreased capacity, they show excellent capacity retention with good rate capability. The rGO/Fe₃O₄ composite obtained at 700 °C in Ar–H₂ exhibits high reversible capacity of 480 mA h g⁻¹ at a high current density of 3000 mA g⁻¹.

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Introduction

In recent years, various oxides containing iron such as Fe₂O₃,^1–4 Fe₃O₄,^5–8 MFe₂O₄ (M = Ni, Co, Mn, Zn),^9–14 etc. have attracted huge interest as anode materials for Li-ion batteries as a replacement for commercial graphite. The lithium storage happens in these oxides via a conversion (redox) mechanism whereby the oxides reversibly react with lithium forming Li₂O along with metal nanoparticles. These oxides have the great advantages of high reversible capacities, environmental benignity, low cost, abundance and slightly lower hysteresis compared to other metal oxides.^2,15 However, their commercialization has been hindered by poor lithium cycling, high operating potentials and large polarization due to poor lithiation/de-lithiation kinetics, which need to be addressed for utilization of these oxides as commercial anodes.

The magnetite phase (Fe₃O₄) is one of the attractive anodes and exhibits a normal spinel structure in which the divalent and trivalent cations occupy tetrahedral and octahedral sites respectively. It undergoes reversible lithium storage via a conversion reaction with reversible formation of Fe nanoparticles in Li₂O matrix. It has a theoretical capacity of 926 mA h g⁻¹, assuming 8 moles of Li storage per mole of Fe₃O₄.

Lithium storage properties of iron oxide were reported to improve with size reduction to nanoparticles, owing to good Li transport as well as the easing of the strain created by the conversion reaction.¹⁶ These improved electrochemical properties of the nanoparticles have motivated the synthesis of Fe₃O₄ at the nanoscale by various routes like solvothermal,^17,18 hydrothermal,^19,20 carbothermal reduction,²¹ sol–gel,²² electrospinning,²³ etc. for application in Li-ion batteries. In addition, different strategies like carbon coating^24–26 and compositing with CNT^27,28 and graphene^29–31 have also been employed for the improvement of electronic conductivity of the material.

In this study, reduced graphene oxide (rGO)-wrapped magnetite nanoparticles were prepared by a simple precipitation method. Precipitation of GO/FeSO₄ solution by addition of NH₄OH, followed by freeze drying of the colloidal mixture resulted in the formation of rGO/Fe₃O₄ nanocomposites. An investigation of the effect of annealing temperature on the electrochemical properties of these composites has found a stable and very high reversible capacity of the composites with good rate capability.

Experimental section

All the chemicals and solvents used in this study are commercially available and were used without further purification.

Synthesis of graphene oxide

Graphene oxide (GO) was prepared according to previous literature.^32,33 Graphite powder (2 g) was added carefully to an 80 °C mixture of conc. H₂SO₄ (3 mL), K₂S₂O₈ (1 g), and P₂O₅ (1 g), followed by stirring for a few minutes until the mixture turned dark blue. Then it was allowed to cool to room temperature over a period of 8 hours. Later, it was carefully diluted with 1 L of distilled water which resulted in partial oxidation of graphite. It was filtered and washed with distilled water until the pH of the filtrate was ∼7. The powder was then dried overnight in air at room temperature.

The partially oxidized graphite powder (2 g) was slowly added to 50 mL of cold (0 °C) conc. H₂SO₄ and 6 g of KMnO₄ was added in small amounts with continuous stirring so that the temperature of the mixture was never allowed to reach 20 °C. After complete addition of KMnO₄, the temperature of the mixture was increased to 35 °C and stirred for another 2 h. Then, 200 mL of distilled water was added carefully. After 15 min, the reaction was terminated by addition of ∼1 L of distilled water and 5 mL of 35% H₂O₂ when the colour of the mixture changed to bright yellow. The mixture was filtered and washed with 1 : 10 HCl solution (500 mL) to remove the residual metal ions. The resulting product was washed and suspended in distilled water to prepare 0.1% (w/v) solution. To prepare GO, the graphite oxide dispersion was exfoliated by sonication in an ultrasonic bath for 30 min. The resulting GO solution was yellowish brown in colour and was stable for a period of a few months.

Synthesis of rGO/Fe₃O₄

The rGO/Fe₃O₄ nanocomposites were prepared by a simple precipitation method coupled with freeze drying. In a typical synthesis, ∼2.8 g (10 mmol) of FeSO₄·7H₂O was dissolved in ∼200 mL of deionized water and ∼50 mL of graphene oxide solution (0.1 wt%) was added and stirred for 12 h to obtain a homogeneous brown solution. During this step, partial oxidation of ferrous to ferric state occurs. Later, ammonium hydroxide was added drop-wise with continuous stirring until the pH of the mixture became neutral, resulting in the formation of a black colloidal solution which was then subjected to freeze drying for 12 h to obtain a porous powder. Freeze drying helps to prevent the aggregation of particles in comparison to normal drying. To prepare the rGO/magnetite nanocomposites, the obtained porous powder was heated under different conditions, namely: (a) 80 °C in air, (b) 600 °C in Ar, (c) 700 °C in Ar and (d) 700 °C in Ar–H₂. The as-prepared rGO/Fe₃O₄ composites were black in colour. CHNS analysis revealed the percentage of rGO in the samples obtained at 80 and 600 °C to be ∼4% while the samples annealed at 700 °C had only ∼3% rGO.

Structural and electrochemical characterization

The rGO/Fe₃O₄ nanocomposites prepared in this study were characterized by powder X-ray diffraction (PXRD) using a Bruker D5005 diffractometer employing graphite monochromatized Cu-Kα radiation (λ = 1.54056 Å). Rietveld refinement of the PXRD patterns was performed using TOPAS software. Raman spectra of the samples were recorded with a Renishaw Raman system 2000. Brunauer–Emmett–Teller (BET) surface area of the samples were determined from N₂ adsorption–desorption isotherms at 77 K using a Tristar 3000 (Micromeritics, USA). The samples were preheated for 2 h at 180 °C under nitrogen flow to remove adsorbed moisture prior to BET analysis. Morphology of the samples was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM micrographs of thin layers of platinum-coated samples were recorded with a JEOL JSM-6700F field emission scanning electron microscope (FESEM) operated at 5 kV and 10 μA. A JEOL JEM 2010 (operated at 200 kV) was used to record the TEM images to determine the surface morphology and the particle size.

Electrochemical properties of the samples were investigated using coin cells (type 2016) with Li metal (Kyokuto Metal Co., Japan) as counter electrode, a glass microfiber filter (GF/F, Whatman International Ltd, Maidstone, England) as the separator and 1 M LiPF₆ in ethylene carbonate, dimethyl carbonate and diethyl carbonate (1 : 1 : 1 v/v, Merck) as the electrolyte. Firstly, a slurry was prepared by mixing the sample with super P carbon black (15 wt%) and PVDF binder, Kynar 2801 (15 wt%) in N-methylpyrrolidone solvent. The slurry was stirred for 12 h and then coated onto an etched copper foil, dried at 80 °C and cut into circular discs of 16 mm in diameter. Coin cells were assembled in an Ar-filled glove box (MBraun, Germany) with oxygen and water concentration maintained below 1 ppm, by crimp sealing the thus fabricated anode with lithium metal as counter electrode. The cells were aged for 8 h before they were subjected to electrochemical testing. Cyclic voltammetry and galvanostatic discharge–charge cycling studies of the cells were carried out at room temperature using a computer controlled MacPile II (Bio-logic, France) and a Bitrode multiple battery tester (model SCN, Bitrode, USA), respectively. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 180 kHz to 0.003 Hz using a Solartron 1260A impedance analyzer.

Results and discussion

Structure and morphology

During the synthesis, addition of NH₄OH to GO/FeSO₄ solution yielded an amorphous black powder which was dried at 80 °C to get the rGO/Fe₃O₄ composite. It was also annealed under different conditions as mentioned earlier to study the effect of annealing temperature on the electrochemical properties. PXRD patterns of the different rGO/Fe₃O₄ composites are shown in Fig. 1. For each sample, all the diffraction peaks match well with the standard pattern (JCPDS card no. 75-0033) and can be indexed to the cubic space group Fd3m. Fe₃O₄ exhibits a normal spinel structure containing iron in a mixed-valent (Fe^2+/3+) state. The divalent Fe²⁺ ions (r_i = 0.74 Å) and the trivalent Fe³⁺ ions (r_i = 0.69 Å) occupy tetrahedral and octahedral sites respectively. The absence of any impurity peaks such as those of other iron oxides (Fe₂O₃ and FeO) indicates the phase purity of the prepared composites. In addition, the absence of GO peaks in the patterns of rGO/Fe₃O₄ composites indicates the reduction of GO to rGO. The sample prepared at 80 °C shows slightly broadened peaks compared to high temperature annealed samples. This indicates an increase in crystallinity and particle size of Fe₃O₄ with an increase in the annealing temperature. Fig. 2 shows the Rietveld refinement of rGO/Fe₃O₄. The experimental pattern matches well with the calculated pattern as evidenced from the difference pattern. The lattice parameter (a) of the different composites was obtained from the Rietveld refinement of respective PXRD patterns (Table S1†). The lattice parameters of rGO/Fe₃O₄ obtained at 80 °C in air, 600 °C in Ar, 700 °C in Ar and 700 °C in Ar–H₂ are calculated to be 8.3665(1), 8.3700(4), 8.3947(9) and 8.3824(6) Å respectively. These values match well with the reported lattice parameter of Fe₃O₄.³⁴

Fig. 1 PXRD patterns of rGO/Fe₃O₄ nanocomposites synthesized under different conditions.

Fig. 2 Rietveld refinement of rGO/Fe₃O₄ (80 °C in air). Blue circles show the experimental pattern while the red and green lines indicate the calculated pattern and the difference between observed and calculated patterns, respectively.

Raman spectroscopy is an important technique to characterize graphene-based materials. Fig. S1† shows the Raman spectra of different rGO/Fe₃O₄ composites recorded at room temperature in the range of 2000–1000 cm⁻¹. Bare GO shows Raman bands at 1600 and 1350 cm⁻¹ which are described as G and D bands respectively. The D band refers to the defects and disordered atomic arrangement due to the sp³-carbon atoms and the vibration of the sp²-carbon atoms in the two-dimensional lattice gives rise to the G band. Upon reduction of GO to rGO, the intensity ratio of the D band to the G band (I_D/I_G) increases. In the case of different rGO/Fe₃O₄ composites, the characteristic G and D bands appear at ∼1580 and ∼1350 cm⁻¹ respectively. The I_D/I_G ratios for the composites are greater than 1 except the sample annealed at 700 °C in Ar which shows a ratio slightly less than 1. This higher I_D/I_G ratio of the rGO/Fe₃O₄ composites indicates the in situ reduction of GO. The G band of the composites shifts by ∼20 cm⁻¹ which indicates the incorporation of Fe₃O₄ nanoparticles into the rGO layers.

Morphology and particle size of the different rGO/Fe₃O₄ nanocomposites were probed using SEM and TEM techniques. Fig. 3 shows the SEM images of the nanocomposites. The sample obtained at 80 °C consists of nanoparticles of size ∼20–50 nm wrapped by rGO layers (Fig. 3b). The rGO layers can be seen clearly from the TEM image (Fig. 4a). The presence of rGO along with the Fe₃O₄ nanoparticles is expected to improve the electronic conductivity compared to bare Fe₃O₄. With an increase in the annealing temperature to 600 °C, the particles form aggregates as seen in Fig. 3d. However, the individual particle size was still ∼50–100 nm. With further increase in the annealing temperature to 700 °C in Ar, the particles fuse together to form bigger particles of ∼50–200 nm in size. Loss of some rGO content at higher temperature also plays a role in the fusion of individual particles.

Fig. 3 Scanning electron micrographs of rGO/Fe₃O₄ nanocomposites annealed at (a & b) 80 °C in air, (c & d) 600 °C in Ar, (e & f) 700 °C in Ar and (g & h) 700 °C in Ar–H₂.

Fig. 4 Transmission electron micrographs of rGO/Fe₃O₄ nanocomposites annealed at (a) 80 °C in air, (b & c) 600 °C in Ar and (d) 700 °C in Ar–H₂.

Nitrogen adsorption–desorption isotherms of the rGO/Fe₃O₄ composites are shown in Fig. 5. The rGO/Fe₃O₄ (80 °C) shows a high BET surface area of 30 m² g⁻¹ though the amount of rGO in the composite was only 4%. This high surface area can be attributed to the small particle size of Fe₃O₄ and the presence of rGO sheets in the composite. The samples annealed at higher temperatures show decreased surface area due to an increase in the particle size, aggregation and slight decrease in the rGO content. The BET surface area, pore size and pore volume of the different rGO/Fe₃O₄ samples are compared in Table S1.†

Fig. 5 Nitrogen adsorption–desorption isotherms of rGO/Fe₃O₄ nanocomposites annealed at 80 °C in air and 700 °C in Ar–H₂.

Galvanostatic cycling

Galvanostatic cycling studies of the different composites were carried out in the potential window 0.005–3.0 V vs. Li/Li⁺ to investigate the Li storage and cyclability. A constant current density of 60 mA g⁻¹ was used for the testing at room temperature. The charge–discharge profiles of the composites for a few selected cycles are shown in Fig. 6a–d.

Fig. 6 Galvanostatic cycling studies showing charge–discharge profiles of rGO/Fe₃O₄ synthesized at (a) 80 °C in air, (b) 600 °C in Ar, (c) 700 °C in Ar and (d) 700 °C in Ar–H₂.

The first discharge curves of all the rGO/Fe₃O₄ composites are different from the remaining cycles indicating a different lithium storage mechanism during the initial discharge. It starts from the open circuit voltage (∼2.9 V) and shows a steep decrease in the voltage to 1.6 V followed by two small plateaus at ∼1.4 and ∼1.0 V. These plateaus correspond to the Li intercalation into Fe₃O₄ forming a lithium intercalated phase, Li_xFe₃O₄ (x ≈ 2), as shown in eqn (1). During this reaction, the two Fe³⁺ ions in Fe₃O₄ are reduced to Fe²⁺. Except for the sample prepared at 700 °C in Ar, this plateau was observed for all the composites. Further decrease in the voltage results in lithiation of the intercalated phase as indicated by a large plateau at ∼0.85 V. During this process, 6 more moles of Li are consumed as per eqn (2), resulting in an irreversible destruction of the crystal structure. This process is accompanied by the reduction of all Fe²⁺ ions to Fe metal nanoparticles which are embedded in an amorphous Li₂O matrix.³⁵ Thus 8 moles of Li are utilized per mole of Fe₃O₄, corresponding to a theoretical capacity of 926 mA h g⁻¹. However, the first discharge capacities observed for rGO/Fe₃O₄ obtained at 80 °C (air), 600 °C (Ar), 700 °C (Ar) and 700 °C (Ar–H₂) were 1855, 1716, 1406 and 1554 mA h g⁻¹ respectively. This additional capacity can be explained by the formation of a solid electrolyte interface (SEI) by the reaction of lithium with the solvents of the electrolyte.^24,36 In addition, rGO present in the samples (3–4%) also contributes to lithium storage and SEI formation.³⁰

During the first charge cycle, lithium is extracted from the material at ∼1.4–2.0 V as indicated by a sloping plateau. The Li₂O and Fe nanoparticles formed during the initial discharge cycle react with each other to form iron oxide according to eqn (3). The first charge capacities obtained for rGO/Fe₃O₄ synthesized at 80 °C in air, 600 °C in Ar, 700 °C in Ar and 700 °C in Ar–H₂ were 1254, 1082, 853 and 1001 mA h g⁻¹ respectively. The second discharge curve for each sample is different from the first discharge cycle. The second and subsequent cycles rely on the reversible conversion of iron oxide to Fe nanoparticles embedded in Li₂O matrix. The voltage of the discharge plateau also shifts to ∼1.0 V compared to 0.85 V observed for the first cycle.

Fe₃O₄ + xLi⁺ + xe⁻ → Li_xFe₃O₄ (1)

Li_xFe₃O₄ + (8 − x)Li⁺ + (8 − x)e⁻ → 3Fe + 4Li₂O (2)

3Fe + 4Li₂O ↔ Fe₃O₄ + 8Li⁺ + 8e⁻ (3)

The conversion reaction happening in the rGO/Fe₃O₄ anodes is found to be highly reversible as inferred from the overlapping charge–discharge curves of the different cycles (Fig. 6). The variation of charge and discharge capacities with cycle number of the different composites and their corresponding coulombic efficiencies are shown in Fig. 7. The sample obtained at 80 °C shows the highest charge capacity of 1254 mA h g⁻¹ for the first cycle which can be attributed to smaller particle size and high surface area. However, the cycling stability was slightly reduced resulting in a capacity of 1046 mA h g⁻¹ at the end of 40 cycles which is still higher than the theoretical capacity. Upon increase in the annealing temperature, the particle size of Fe₃O₄ increases and hence the surface area decreases dramatically. The sample obtained at 600 °C in Ar shows a slightly lesser capacity of 1082 mA h g⁻¹ for the first cycle. However, it exhibits better cycling stability with a capacity of 1062 mA h g⁻¹ obtained at the end of 40 cycles.

Fig. 7 Capacity vs. cycle number plots of rGO/Fe₃O₄ nanocomposites synthesized at (a) 80 °C in air, (b) 600 °C in Ar, (c) 700 °C in Ar and (d) 700 °C in Ar–H₂.

With further increase in annealing temperature to 700 °C, the capacity decreases further due to slightly decreased graphene content and increased particle size. The charge capacity obtained for the samples annealed at 700 °C in Ar and Ar–H₂ were 853 and 1001 mA h g⁻¹. However, capacity retention of these samples was found to be excellent with reversible capacity of 854 and 972 mA h g⁻¹ obtained after 40 cycles. In addition, all the composites possess a high coulombic efficiency of ∼99%. The details of the charge and discharge capacities of the individual samples for the 1^st, 2^nd and 40^th cycles are listed in Table 1. Though the reversible capacities obtained for these composites are quite high and stable, high polarization of ∼0.6 V is observed which is characteristic of these oxides. Though many reports on rGO/Fe₃O₄ appear in the literature, the graphene/rGO contents in the samples were very high leading to good capacity.^37–40 In this study, better electrochemical performance was achieved for the rGO/Fe₃O₄ samples, even though the rGO content was only 3–4%.

Table 1 Charge and discharge capacities of rGO/Fe₃O₄ nanocomposites synthesized under different conditions

Compound	Discharge capacity (mA h g^−1)			Charge capacity (mA h g^−1)
	1^st cycle	2^nd cycle	40^th cycle	1^st cycle	2^nd cycle	40^th cycle
rGO/Fe3O4 80 °C in air	1855	1236	1047	1254	1127	1046
rGO/Fe3O4 600 °C in Ar	1716	1105	1080	1082	1063	1062
rGO/Fe3O4 700 °C in Ar	1406	911	865	853	869	854
rGO/Fe3O4 700 °C in Ar–H2	1554	978	977	1001	960	972

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Rate capability studies

High rate capability is an important characteristic of a good electrode material. Rate capability studies help in understanding the ability of a material to store lithium at different current rates. A material with a good capacity at high current rates is very useful as it can be charged and discharged in a short time. All the different rGO/Fe₃O₄ composites were subjected to galvanostatic charge–discharge cycling at different current densities of 500, 1000, 2000 and 3000 mA g⁻¹ which correspond to current rates of ∼0.5, 1.1, 2.2 and 3.2 C respectively (1 C represents a current density of 926 mA g⁻¹). As expected, the specific capacity of the composites decreases when the current density is increased from 60 mA g⁻¹ to higher current densities. Fig. 8 shows the capacity vs. cycle number plots at different current rates for the rGO/Fe₃O₄ composites synthesized under different conditions. The sample obtained at 600 °C in Ar shows the best performance at 500 and 1000 mA g⁻¹. However, at higher rates, the reversible capacity obtained was poor. The sample annealed at 700 °C in Ar–H₂ shows the best capacity retention at high current rates. This can be attributed to the better reduction of rGO in a reducing Ar–H₂ environment. When the current density was decreased from 3000 to 60 mA g⁻¹, the capacity obtained was found to be higher than the original capacity obtained at 60 mA g⁻¹. This increase in capacity can be attributed to the increase in the activity of graphene upon cycling, as evidenced by the large capacity obtained after the plateau region (Fig. S2†).

Fig. 8 Rate capability of rGO/Fe₃O₄ nanocomposites synthesized under different conditions.

Cyclic voltammetry

Cyclic voltammetry (CV) is a complementary technique to galvanostatic cycling which gives information about the redox potentials of electrochemical reactions occurring in the electrodes. Fig. 9 shows the cyclic voltammograms of the different rGO/Fe₃O₄ composites for selected cycles. The experiments were carried out at room temperature using Li metal as the counter electrode in the potential window of 0.005–3 V at a constant scan rate of 58 μV s⁻¹. All four samples show similar CV curves and their first cycle exhibits a substantial difference from the subsequent cycles. During the first cathodic scan (reduction or Li insertion), two small peaks at ∼1.4 and ∼0.8 V were observed which are due to the formation of the lithium intercalated compound Li_xFe₃O₄ from the magnetite phase (Fe₃O₄) as per eqn (1).

Fig. 9 Cyclic voltammograms of rGO/Fe₃O₄ synthesized at (a) 80 °C in air, (b) 600 °C in Ar, (c) 700 °C in Ar and (d) 700 °C in Ar–H₂.

In the case of the sample annealed at 700 °C in Ar, the peak at 0.8 V did not appear, similar to the galvanostatic cycling results. Upon further discharge to the cut-off voltage of 0.005 V, a sharp peak appears at ∼0.5–0.6 V, which corresponds to crystal structure destruction of Fe₃O₄ forming Fe nanoparticles embedded in Li₂O matrix. This peak was not seen in the subsequent cycles, indicating the irreversible nature of the first discharge cycle (eqn (2)). The first anodic scan shows two overlapped peaks at ∼1.7 V which correspond to the oxidation of Fe to form Fe₃O₄ (eqn (3)). In the subsequent cycles, a single broad peak was observed at 0.7–0.8 V for the different composites during cathodic scans and at 2.0 V during the anodic scans. Similar redox potentials have been reported for magnetite anode materials. The overlapping CV curves also indicate the excellent reversibility of lithium cycling in the materials.

Electrochemical impedance spectroscopy

EIS is an important technique used to analyse electrode kinetics. The impedance measurements were carried out for the different rGO/Fe₃O₄ nanocomposites for the 1^st and 40^th cycles at selected voltages. During voltage increment (charging) and decrement (discharging), the cell was subjected to a current density of ∼200 mA g⁻¹ and was relaxed at the given voltage for 2 h before data collection. The results are plotted as Nyquist plots (Z_re vs. Z_im), where Z_re and Z_im are respectively the real and imaginary parts of cell impedance.

Fig. S3a† compares the Nyquist plots obtained at open circuit voltage for all the different composites. They show a single semicircle with an overall impedance of ∼220–250 Ω except for the 80 °C sample which shows only 120 Ω. This large semicircle is attributed mainly to the high surface film resistance R_sf and the associated capacitance, CPE_sf. With the onset of lithium insertion, the surface film resistance is expected to decrease and the charge transfer resistance increases. The overall impedance has been observed to be ∼60–80 Ω at the fully discharged state (0.005 V) as shown in Fig. S3b† where the Fe₃O₄ particles are disintegrated to Fe metal nanoparticles in Li₂O matrix.

Detailed EIS measurements were carried out for the rGO/Fe₃O₄ composite prepared at 700 °C in Ar–H₂. EIS data were collected for the sample at different voltages for the 1^st charge and discharge cycles as shown in Fig. 10. During the discharge cycle, the impedance decreases with the commencement of lithiation and reaches ∼70 Ω at the fully discharged state. During the charge cycle, the impedance was very low (15–20 Ω) in the plateau region indicating good conductivity of the sample.

Fig. 10 Nyquist plots of rGO/Fe₃O₄ (700 °C Ar–H₂) at different voltages for (a) 1^st discharge cycle and (b) 1^st charge cycle.

The composites were also subjected to 40 charge–discharge cycles and the EIS data were recorded at the discharged state (Fig. S3c†). As can be seen, the impedance was very low, ∼20 Ω, for all the samples except the sample annealed at 600 °C in Ar. This explains the good cycling stability of the composites. The fast capacity fading of the sample prepared at 600 °C in Ar can be explained by its high impedance compared to the other samples.

Conclusions

In summary, porous rGO-wrapped magnetite nanoparticles were obtained by a simple precipitation reaction followed by freeze drying and heating at 80 °C in air. The sample with ∼4% rGO has a high surface area of 30 m² g⁻¹ and exhibits a high reversible capacity of over 1200 mA h g⁻¹ at a current density of 60 mA g⁻¹. However, the capacity retention was not quite as good. Annealing the composites at higher temperatures leads to increased particle size and decreased surface area. However, these composites exhibit very good lithium cycling properties and minimal capacity fading. The sample annealed at 700 °C in Ar–H₂ atmosphere exhibits the best rate capability with a capacity of ∼480 mA h g⁻¹ at a high current rate of 3.2 C (3000 mA g⁻¹).

Acknowledgements

We thank the Ministry of Education for funding this project through NUS FRC grant no. R-143-000-562-112.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11948g

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