FeS2 microspheres wrapped by N-doped rGO from an Fe-based ionic liquid precursor for rechargeable lithium ion batteries

Xueda Ding ab, Chengfeng Du *c, Jianrong Li *a and Xiaoying Huang a
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 350002 Fuzhou, P.R. China. E-mail: jrli@fjirsm.ac.cn
bCollege of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, PR China
cState Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, P.R. China. E-mail: cfdu@nwpu.edu.cn

Received 2nd November 2018 , Accepted 21st November 2018

First published on 21st November 2018


Earth-abundant pyrite (FeS2) is a promising anode material for lithium ion batteries (LIBs) because of its high theoretical specific capacity (894 mA h g−1). However, LIBs using pristine FeS2 usually suffer from volume expansion, dissolution of polysulfides, and low conductivity of Li2S. Herein, FeS2/N-doped reduced graphene oxide microspheres (FeS2/N-rGO) are first synthesized from an Fe-based ionic liquid, [C12MMim]FeCl4 (C12MMim = 1-dodecyl-2,3-dimethylimidazolium), which can not only be used as the metal and nitrogen source but also as an assembly medium and surfactant. As the anode material for rechargeable LIBs, the as-obtained FeS2/N-rGO composites display a specific capacity of 950 mA h g−1 after 140 cycles at a current density of 150 mA g−1 and deliver an average reversible discharge capacity of 973, 867, 778, and 671 mA h g−1 at 0.2, 0.5, 1.0, and 2.0 A g−1, respectively. Even at high current density, the specific capacity can still reach 510 mA h g−1. More importantly, after deep cycling, a high reversible capacity of 973 mA h g−1 can still be recovered when the current density reduced to 0.2 A g−1. This excellent stability and outstanding rate performance are mainly attributed to the suppression of dissolution of polysulfide intermediates and volume expansion by the conductive N-doped rGO matrix.


Introduction

With the ever-increasing demand for portable electronics and the rapid development of electric vehicles, renewable energy storage devices including rechargeable lithium-ion batteries (LIBs) with high energy density, reliable safety, and satisfactory cycling performance are highly required.1–4 Up to now, the energy densities of commercial LIBs with traditional LiMO2 (M = transition metal) cathodes and graphite anodes have approached the practical upper limit.5–7 Pyrite (FeS2) with the advantages of being earth-abundant, environmentally benign, and affordable is a promising anode material candidate for LIBs.8,9 More importantly, the four-electron reduction of cubic-FeS2 by lithium (FeS2 + 4Li+ + 4e → Fe + 2Li2S) offers a high theoretical specific capacity of 894 mA h g−1,10 which is superior to that of the very best conventional LiMO2 intercalation cathodes.11 However, during the process of discharge, highly reactive Fe0 particles and insulating Li2S will be generated through the conversion reactions of FeS2. Because of the insulating characteristic of Li2S, the dissolution of polysulfides Li2Sx (2 < x < 8) converted from the in situ generated sulfur occurs, and the highly reactive Fe0 particles tend to aggregate into non-reactive α-Fe0 particles, and an undesired cycling performance and low capacity retention are normally obtained.12–14 Numerous research studies have been devoted to the design of novel structures, construction of conductive polymer coatings, and synthesis of FeS2–carbon composites to optimize the electrochemical performance of FeS2 for LIBs.15–24 Notably, reduced graphene oxide (rGO) has attracted much attention in combination with FeS2 to enhance the stability and energy density of LIBs owing to its fascinating features of superior electric conductivity, chemical stability, and high surface area, which can effectively alleviate the volume expansion during Li+ insertion–extraction.22–24 Recently, heteroatom (B, N, P, S) doped rGO derivatives have been demonstrated to possess more heteroatom defects and higher conductivity, and as a consequence, the conductivity of these materials is further improved.25–30

In recent years, ionic liquids (ILs) have attracted increasing attention because of their ability to be a promising green alternative to organic solvents in various synthetic processes, as well as other fascinating applications.31–33 Interestingly, ILs can form expanded hydrogen-bond systems in the liquid state and exhibit a remarkable ability to assemble various nanostructures owing to the existence of abundant non-covalent interactions like electrostatic interactions, van der Waals forces, hydrogen bonding, and π–π stacking.34 In particular, ILs can act as a solvent, metal source, and morphology template for synthesizing hierarchical structures, and this “all-in-one” synthetic strategy not only effectively simplifies the synthesis system but also reduces the use of structure-directing agents in the construction of some substances with special morphologies. Until now, many functional materials like 3D hierarchical CuS microspheres,35 ferric giniite spheres,36 CuSe nanoflakes,37 α-Fe2O3 cubes,38 and sponge-like iron-based fluoride cathodes39 have been successfully synthesized adopting the “all-in-one” ILs. Just as expected, the orientation arrangement and the surface adsorption of ILs have an important influence on the final morphology of these functional materials.

Herein, an Fe-based ionic liquid, [C12MMim]FeCl4 (C12MMim = 1-dodecyl-2,3-dimethylimidazolium), was chosen as a multifunctional precursor for the synthesis of nitrogen-doped reduced graphene oxide wrapped FeS2 microsphere composites (denote as FeS2/N-rGO). In these composites, FeS2 was wrapped by the N-doped rGO matrix and further assembled into microspheres. The doped nitrogen atoms derived from the imidazolium group of the Fe-based ionic liquid further enhance the conductivity of rGO. As a result, the obtained FeS2/N-rGO composites as anode material exhibit high reversible capacity and outstanding rate capability. Hence, this work offers an effective paradigm for preparing FeS2/N-doped rGO composites from an Fe-based ionic liquid precursor for rechargeable lithium ion batteries.

Experimental section

Synthesis of the Fe-based ionic liquid

[C12MMim]FeCl4 was prepared according to our previous report.40 In a typical synthesis, 10 mmol of [C12MMIm]Cl (3.010 g) and 10 mmol of FeCl3·6H2O (2.705 g) were mixed in a 25 mL round-bottomed flask with the addition of reverse osmosis (RO) water (10 mL), followed by magnetically stirring for 30 min in a water bath (30 °C). The resulting mixture was dried at 60 °C for 24 h under dynamic vacuum. Then [C12MMIm]FeCl4 was obtained.

Synthesis of graphene oxide (GO)

GO was prepared via a modified Hummers method.41 In a typical synthesis, a 9[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture of concentrated H2SO4/H3PO4 (360[thin space (1/6-em)]:[thin space (1/6-em)]40 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt equiv.) and KMnO4 (18.0 g, 6 wt equiv.) in an ice water bath. The reaction was then heated to 50 °C and stirred for 12 hours in an oil bath. The reaction was cooled to room temperature and poured onto ice (400 mL). 30% H2O2 was then added dropwise to the above solution until the color of the solution turned from purple to golden yellow. The obtained solution was washed by centrifugation at 9000 rpm for 1 hour with HCl (0.1 mol L−1) and RO water individually several times until the pH of the solution reached 7.0. Finally, the GO suspension was obtained by washing with alcohol several times via centrifugation.

Synthesis of FeS2/N-doped rGO composites (FeS2/N-rGO)

Firstly, 50 mL of GO ethanol solution with different concentrations (0.5 mg mL−1, 1 mg mL−1 and 2 mg mL−1) were added to [C12MMim]FeCl4 (0.7 mmol) under stirring for 2 hours. Then, thiourea (6 mmol) was added to the above solution with stirring for 0.5 hours. Finally, the solution was transferred into a 100 mL Teflon-lined autoclave and heated at 190 °C for 20 hours. The samples were centrifuged and the resulting precipitate was washed with RO water and absolute alcohol several times, and then dried in a vacuum oven at 60 °C for 8 hours. The final products were annealed in Ar gas at 300 °C for 2 hours, denoted as FeS2/N-rGO-0.5, FeS2/N-rGO-1 and FeS2/N-rGO-2, respectively. For comparison, the FeS2/reduced GO composite was synthesized under the same conditions with the 1 mg mL−1 GO except for using FeCl3·6H2O instead of [C12MMim]FeCl4, denoted as FeS2/rGO.

Materials and instrumentation

Thiourea (99.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd. All the reagents and solvents were commercially available and used as received without further purification. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Miniflex II diffractometer by using Cu-Kα radiation (λ = 1.54184 Å). Thermogravimetric analysis (TGA) was carried out on an STA449C unit from room temperature to 1000 °C in an air atmosphere. Energy-dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) analyses were performed with a JEOL JSM-6700F instrument, Hitachi FE-SEM SU8010, and US FEI company thermal field launch QUANTA Q400. Transmission electron microscopy (TEM) images were obtained with a JEM-2010F instrument. The Raman spectra of the samples were collected using a LabRAM HR spectrophotometer equipped with a microscope having a laser of wavelength 532 nm. X-ray photoelectron spectroscopy (XPS) spectra were measured using an ESCALAB250Xi. Electrochemical measurements were carried out using a CHI650e electrochemical workstation (Shanghai Chenhua Instrument, China).

Electrochemical measurements

The electrochemical tests were performed using CR2025 coin-type test cells. The cells were assembled in a high-purity Ar-filled glove box in which the concentrations of moisture and oxygen were below 1 ppm. The working electrodes were prepared by mixing 80 wt% active materials, 10 wt% conductive agent (TIMCAL SUPER C45 Carbon Black), and 10 wt% polyvinylidene fluoride binder (PVDF) in 1-N-methyl-2-pyrrolidinone (NMP) to obtain a homogeneous slurry. The above slurry was coated on copper foil and then dried at 60 °C for 6 hours followed by 120 °C for 15 hours in a vacuum oven. After that the copper foil was cut into Φ 1.6 cm disks with the mass loading of active materials around 1.2 mg cm−2. Lithium metal was used as the counter/reference electrode, Celgard 2325 membrane as the separator and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 by volume) as the electrolyte solution. The galvanostatic charge/discharge cycles were carried out on a LAND 2001A system over a voltage range of 0.05 to 3.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a CHI650E Electrochemical Workstation. The CV was performed at a scan rate of 0.1 mV s−1 within a potential window of 0.05–3.0 V. The EIS spectra were recorded in constant voltage mode over the frequency range between 100 kHz and 0.01 Hz.

Results and discussion

The FeS2/N-doped rGO composites were prepared by a solvothermal reaction coupled with thermal treatment, which is schematically illustrated in Fig. 1. Firstly, the Fe-based ionic liquid [C12MMim]FeCl4 (its structure is shown in Fig. S1) was dispersed in the as-prepared GO suspension solution and stirred to form a homogeneous mixture. The negatively charged GO has abundant oxygen-containing functional groups that can adsorb positively charged [C12MMim]+ released from the Fe-based ionic liquid through electrostatic and π–π interactions,34,42 which can effectively prevent the accumulation of graphene sheets. After adding thiourea, H2S was generated by the hydrolytic reaction of thiourea and it reacted with FeCl4 to form FeS2 crystals. The [C12MMim]+ can also serve as a stabilizer to prevent the agglomeration of FeS2 by adsorbing on its surface. The FeS2 wrapped by GO sheets will self-assemble into microspheres to reduce the surface energy. Meanwhile, GO was reduced to rGO by the high pressure and temperature during the solvothermal process. After pyrolysis in Ar gas, nitrogen atoms from the residual [C12MMim]+ were doped into rGO, resulting in the FeS2/N-doped rGO composites. The obtained samples were prepared using 0.5, 1 and 2 mg mL−1 GO suspension solution denoted as FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2, respectively. For comparison, FeS2/rGO composites were prepared by using FeCl3·6H2O instead of [C12MMim]FeCl4 with 1 mg mL−1 GO suspension solution under the same conditions.
image file: c8se00539g-f1.tif
Fig. 1 Schematic illustration of the formation process of FeS2/N-rGO composites.

Fig. 2a depicts the PXRD patterns of various samples and the crystal structure of FeS2. All the diffraction peaks can be assigned to the standard FeS2 (PDF#42-1340). Besides, the redundant characteristic diffraction peak at about 26° can be indexed to rGO. Pyrite FeS2 has a cubic structure with the Pa3 space group, and the unit cell consists of an Fe face-centred cubic sublattice, in which the S atoms are embedded.43Fig. 2b and c display the scanning electron microscopy (SEM) images of FeS2/N-rGO-1; it can be clearly seen that the surfaces of these microspheres with a diameter of 1–2 μm are tightly wrapped by rGO sheets. The obtained FeS2/N-rGO-0.5 and FeS2/N-rGO-2 also exhibit similar morphologies (Fig. S2). The N-doped rGO in these composites can not only behave as a buffer layer to alleviate the volume change caused by the Li+ insertion–extraction and enhance the electrical conductivity, but also limit the dissolution of polysulfides, which can effectively improve the electrochemical performance of these composites for rechargeable LIBs.44,45 When FeCl3·6H2O was selected as the metal source, FeS2/N-rGO microspheres with a diameter of 3–5 μm are also obtained (Fig. S3); however, the rGO sheets coated on their surface were partially aggregated. The microstructure of FeS2/N-rGO-1 was further investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Fig. 2d shows the TEM image of a microsphere with N-doped rGO sheets wrapped on its surface. As shown in the HRTEM image of FeS2/N-rGO-1 (Fig. 2e), the lattice spacing of 0.27 nm is consistent with the (200) planes of FeS2 (PDF#42-1340). The EDS for FeS2/N-rGO-1 (Fig. S4) confirms the presence of Fe, S, C, and N elements within FeS2/N-rGO-1, and the SEM-EDS elemental mapping (Fig. S5) indicates the uniform distribution of Fe, S, C, and N elements.


image file: c8se00539g-f2.tif
Fig. 2 (a) PXRD patterns of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2, and the crystal structure of FeS2. (b and c) SEM images, (d) TEM image, and (e) HRTEM image of FeS2/N-rGO-1.

Fig. 3a presents the results of thermogravimetric analysis (TGA) under an air atmosphere. The commercial FeS2 exhibits a final weight loss of 33.8% at 1000 °C, which is consistent with the theoretical weight loss (33.5%), assuming that the final residue was Fe2O3. Similarly, it can be calculated that the mass content of FeS2 in the FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 composites is around 69.7%, 80.7%, 66.9%, and 56.8%, respectively. Raman spectra of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 are shown in Fig. 3b. The three weak peaks at 338, 380, and 426 cm−1 match well, respectively, with the Eg, Ag, and Tg modes of FeS2.46 The two strong peaks at 1335 cm−1 and 1589 cm−1 correspond to the D band and the G band of rGO, respectively.47 The chemical valence states of FeS2/N-rGO-1 were investigated by X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum of FeS2/N-rGO-1 confirms the existence of C, N, O, S, and Fe elements in FeS2/N-rGO-1 (Fig. 4a), which is consistent with the EDS spectroscopy (Fig. S4). The C 1s peak centered at 284.5 eV is ascribed to the presence of the C–C group (Fig. 4b). The fitted peak observed at the higher binding energy indicates the presence of oxygen-containing functional groups.48 In the S 2p XPS spectrum (Fig. 4c and S6a), the peaks located at 162.6 and 163.8 eV correspond to the S22− species, while the peak located at 168.9 eV corresponds to S in the rGO matrix.49 In the Fe 2p spectrum (Fig. 4d and S6b), the binding energies of 707.1 eV for Fe 2p3/2 and 720 eV for Fe 2p1/2 are the spin–orbit characteristics of Fe2+, while the binding energies of 711.3 and 725.1 eV are possibly attributed to the Fe3+ species from surface oxidation.50 The high-resolution N 1s profiles (Fig. 4e and S7) were best fitted into three contributions, including pyridinic N (398.5 eV), pyrrolic N (399.7 eV), and graphitic N (401.1 eV),25 which confirmed that N has been successfully doped into rGO. In addition, FeS2/N-rGO-1 exhibits the highest proportion of pyridinic-N (32.6%) compared to FeS2/N-rGO-0.5 (29.4%) and FeS2/N-rGO-2 (19.0%) (Fig. 4f). Previous reports have illustrated that pyridinic N possesses the highest activity for association of Li atoms owing to the favourable electron reduction of pyridinic N.51 These analysis results indicate the formation of FeS2/N-rGO composites.


image file: c8se00539g-f3.tif
Fig. 3 (a) TGA curves and (b) Raman spectra of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2.

image file: c8se00539g-f4.tif
Fig. 4 XPS spectra of FeS2/N-rGO-1. (a) Survey scan, (b) C 1s spectra, (c) S 2p spectra, (d) Fe 2p spectra, and (e) N 1s spectra; (f) the distribution of pyridinic-N, pyrrolic-N, and graphitic-N of FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2.

The detailed Li-ion storage performances of FeS2/N-rGO composites as the anode of lithium batteries were characterized. All the FeS2/rGO and FeS2/N-rGO composites display similar CV curves (Fig. 5a and S8). Therefore, only the results of FeS2/N-rGO-1 are described in greater detail herein. The CV curves for the first three cycles of FeS2/N-rGO-1 at a scan rate of 0.1 mV s−1 in the potential range of 0.05–3.0 V (vs. Li+/Li) are shown in Fig. 5a. In the first cathodic scan, the reduction peaks located around 0.70 and 1.2 V are due to the formation of a solid electrolyte interface (SEI) layer on the electrode surface, and the insertion of Li+ into FeS2 (eqn (1)).13 In the anodic scan, the oxidation peaks are located at 1.9 and 2.5 V, corresponding to the formation of Li2−xFeS2 (eqn (2) and (3)).52 In the following reverse cycles, the reduction peaks appear at 1.4 and 2.0 V, which can be attributed to the reactions shown in eqn (4) and (5).53

 
FeS2 + 4Li+ + 4e → 2Li2S + Fe(1)
 
Fe + 2Li2S → Li2FeS2 + 2Li+ + 2e(2)
 
Li2FeS2 → Li2−xFeS2 + xLi+ + xe (0 < x < 0.8)(3)
 
Li2−xFeS2 + 2Li+ + 2e → Fe + 2Li2S(4)
 
FeSy + (2 − y)S + (2 − x)Li+ + (2 − x)e → Li2−xFeS2(5)


image file: c8se00539g-f5.tif
Fig. 5 (a) CV curves at a scan rate of 0.1 mV s−1 and (b) charge/discharge curves of FeS2/N-rGO-1 in the voltage range of 0.05–3.0 V. (c) Cycling performance and (d) rate capabilities at various current densities from 0.2 A g−1 to 5 A g−1 of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, FeS2/N-rGO-2, and the corresponding CE of the FeS2/N-rGO-1 electrode. Kinetic analysis of the FeS2/N-rGO-1 anode: (e) CV curves at scan rates from 0.1 to 2 mV s−1. (f) CV peak current (Ip) logarithmically plotted as a function of sweep rate (v) to give the slope (b).

The first three charge/discharge curves at 150 mA g−1 in the voltage range of 0.05–3.0 V are depicted in Fig. 5b and S9; the potential plateaus in the charge/discharge process match well with the CV curves. There is a long voltage plateau near 1.5 V in the initial discharge cycle and is reduced in the following cycles, reflecting the initial irreversible capacity loss mainly due to the formation of a SEI film.54 The initial CEs for FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 electrodes are 69.0, 74.2, 76.6, and 73.8%, respectively. The low initial Coulombic efficiencies (CEs) and inevitable capacity loss during the first cycle may be due to the formation of the SEI layer and the inadequate decomposition of Li2S.54 After the 7th cycle, the CE of FeS2/N-rGO-1 increased to 99.0% (Fig. 5c), indicating almost complete reversibility during the delithiation and lithiation processes. Fig. 5c shows the cycling performances of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 at a current density of 150 mA g−1 in the voltage range 0.05–3.0 V. It can be seen that the initial discharge specific capacity of the FeS2/rGO electrode is 1230.5 mA h g−1. As the discharge cycle proceeds, the specific capacity of FeS2/rGO decreases to 275.7 mA h g−1 after 140 cycles. Compared with the FeS2/rGO electrode, FeS2/N-rGO-0.5 and FeS2/N-rGO-1 electrodes display similar initial discharge specific capacities, and their values are 1153.9 and 1248.3 mA h g−1, respectively. However, the specific capacities remained at 824.5 and 950 mA h g−1 for the FeS2/N-rGO-0.5 and FeS2/N-rGO-1 electrode after 140 cycles. The higher specific capacity retention of FeS2/N-rGO-0.5 and FeS2/N-rGO-1 is attributed to the suppression of dissolution of polysulfide intermediates by N-doped rGO. Especially, FeS2/N-rGO-1 exhibits the highest specific capacity with a CE of nearly 100% in the subsequent cycles (Fig. 5c), suggesting that FeS2/N-rGO-1 possesses excellent cycling stability. Interestingly, the capacity of FeS2/N-rGO-1 increases from 740 to 950 mA h g−1 within 140 cycles, which has been normally observed for transition-metal compounds.11,22 With further increasing the content of rGO, FeS2/N-rGO-2 exhibits an initial specific capacity of 1091.4 mA h g−1, and it remained at 801.1 mA h g−1 after 140 cycles, which is lower than that of FeS2/N-rGO-0.5 and FeS2/N-rGO-1. This is mainly ascribed to the excessive rGO with low specific capacity which can lead to the decrease in the specific capacity of the FeS2/N-rGO-2 composite.55

Fig. 5d depicts the rate performances of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 at different current densities. FeS2/N-rGO-1 delivers average reversible discharge capacities of 973, 867, 778, and 671 mA h g−1 at various current densities from 0.2, 0.5, 1, to 2 A g−1, respectively. Although the discharge capacity of the FeS2/N-rGO-1 electrode declines in the first few cycles at 0.2 A g−1, it remained stable in the subsequent cycles. Even at a high current density of 5 A g−1, the specific capacity can still reach 510 mA h g−1. More importantly, even after deep cycling at 5 A g−1, a high stable reversible capacity of 973 mA h g−1 can still be recovered when the current density reduced to 0.2 A g−1, indicating that the FeS2/N-rGO-1 composite possesses high electrochemical reaction stability. Fig. S10 depicts the charge/discharge curves of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 at various current densities. The voltage platform of FeS2/rGO almost disappeared at high current densities, in contrast, the voltage platforms of FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 were still present. Such a high electrochemical performance of FeS2/N-rGO-1 is much better than or comparable to that of other previously reported FeS2-based materials for LIBs; the detailed comparison information is summarized in Table S1.

To further investigate the capacitive behaviour of FeS2/rGO, FeS2/N-rGO-0.5, FeS2/N-rGO-1, and FeS2/N-rGO-2 composites, their kinetics were also analyzed by CV measurements (Fig. 5e and f and S11). The CV curves of the electrodes at different scan rates from 0.1 to 2 mV s−1 exhibit a similar shape. The relationship between peak current (i) and scan rate (v) obeys the following equation:

 
i = avb(6)
where i is the peak current, v is the scan rate, and a and b are constants for empirical parameters. Previous research studies suggest that a b-value of 1 indicates an ideal capacitive behaviour, and a b-value of 0.5 exhibits a diffusion controlled process.56,57 In this work, the b-values of FeS2/N-rGO-1 for two cathodic peaks and two anodic peaks can be calculated as 0.94, 0.75, 0.72, and 0.80, respectively. All these values of b reveal a relatively fast kinetics owing to the pseudocapacitive effect.56 The electrochemical impedance spectroscopy (EIS) spectra at the first cycle in the frequency range from 100 kHz to 0.01 Hz of FeS2/rGO and FeS2/N-rGO-1 are depicted in Fig. S12, which indicate that the semicircle diameter of FeS2/N-rGO-1 is much smaller than that of FeS2/rGO, demonstrating the relatively high electrical conductivity for FeS2/N-rGO-1. The favourable conductivity of FeS2/N-rGO-1 is mainly attributed to the N-doped rGO matrix. Benefiting from these advantages mentioned above, FeS2/N-rGO-1 exhibits high reversible capacity and outstanding rate capability for LIBs.

Conclusions

In summary, we have constructed FeS2 microspheres wrapped by N-doped rGO (FeS2/N-rGO) from an Fe-based ionic liquid precursor via a solvothermal reaction coupled with a subsequent pyrolyzation process. The resulting FeS2/N-rGO composites are tightly wrapped by N-doped rGO, which can enhance electrical conductivity, effectively alleviate volume expansion, and inhibit the dissolution of polysulfides, resulting in a high cycling performance and favourable rate performance for rechargeable LIBs. The presented simple synthesis strategy opens up a green and facile method to synthesize other transition metal sulfide composites for LIBs and other energy storage systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the NNSF of China (no. 21371001 and 21521061) and the NSF of Fujian Province (Grant 2016J01083) is greatly acknowledged.

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Footnote

Electronic supplementary information (ESI) available: SEM, SEM-EDS, XPS, CV, charge/discharge curves, and EIS. See DOI: 10.1039/c8se00539g

This journal is © The Royal Society of Chemistry 2019