Fabrication of a reversible SnS₂/RGO nanocomposite for high performance lithium storage

Abstract

SnS₂/graphene (SnS₂/G) composites have been explored extensively as a promising candidate for Lithium Ion Battery (LIB) anodes in recent years. Previously, the SnS₂ conversion/reduction step of the reaction mechanism is generally believed to be irreversible or only partially reversible, which severely underestimates the theoretical capacity of SnS₂. In this work, SnS₂ nanoparticles have been successfully stacked on reduced graphene oxide (RGO) via a facile and effective solvothermal method using ethylene glycol as a chelant. The SnS₂/graphene nanocomposite retained many of the original 2D characteristics of the graphene nanosheets. As a result, Li⁺ storage properties were significantly improved. The SnS₂/RGO nanocomposites show a higher storage capacity of 939.0 mA h g⁻¹ after 30 cycles at a current density of 0.1 A g⁻¹, and a long-term cycle capacity of 615.5 mA h g⁻¹ even after 200 cycles at 1 A g⁻¹. The superior cycling stability of the SnS₂/RGO electrode is attributed to greater reversibility in the initial conversion reaction, ascribed to the presence of the Sn nanoparticles.

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1. Introduction

In recent years, there have been studies on the preparation of various layered structure materials, such as SnS₂, SnS, MoS₂, WS₂, etc. for use in battery applications.^1–13 Among them, SnS₂ has been actively investigated as a possible candidate for anode materials in LIBs due to its relatively high theoretical capacity and CdI₂-type layered crystal structure.^2–4,14,15 The proposed traditional lithium storage reaction mechanism in SnS₂ is as follows:^2,5,15–17

SnS₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂S (1)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) (2)

Reaction (1) of the reaction mechanism has generally been considered to be irreversible. As such, the theoretical capacity of SnS₂, calculated based on Li/Sn stoichiometry of 4.4 in reaction (2), is only as 645 mA h g⁻¹.^2,18

However, the theoretical capacity of SnS₂ calculated based on the theoretical maximum Li/Sn stoichiometry of 8.4 can be as high as 1231 mA h g⁻¹ if Li₂S can be decomposed completely.¹⁹ This motivates the exploration of new Li storage mechanism to realize the higher and reversible capacity. Also, the charge/discharge process of pure SnS₂ possesses several disadvantages that result from the large volume expansion, such as the fast capacity decay, low capacity utilization, poor cycling stability and inferior rate capacity.^2,5,14,20 To avoid the above mentioned disadvantages, some previous research has demonstrated the further enhancement of electrochemical properties, the electrical conductivity and structural stability of the system by combining graphene with metal sulfide hydrate material.^21–24 There are, however, several issues remaining in the said synthetic method, such as the toxicity of hydrazine hydrate and sodium borohydride, the need for high temperature in H₂/Ar gas and low load rate on graphene.^21–23 Further, the reversibility of the conversion reaction (1) is often neglected. Therefore, there are still considerable challenges for improvements in the design and optimization of anode composition to promote the reversible storage of Li⁺.³

Herein, we report a facile, green, one-step solvothermal synthesis of reversible SnS₂/RGO nanocomposites with improved reversibility in Li⁺ storage. Unlike previous reports, our route has fully utilized the advantages of ethylene glycol to achieve good nanoparticle morphology and large loading on RGO nanosheets for an increase in the reversible capacity. The possible proposed chemical reactions are illustrated in Scheme 1. As anodic materials for LIBs, it is expected that SnS₂/RGO nanocomposites show excellent electrochemical performance in Li⁺ storage. SnS₂/RGO delivers a discharge capacity value of about 939.9 mA h g⁻¹ after 30 cycles and allows the reversible storage and retrieval of 6.2Li⁺ per Sn atom under a charge capacity of 914.6 mA h g⁻¹ after 30 cycles. Therefore, the reversibility of the conversion reaction, as stipulated in step (1), Sn + 2Li₂S → SnS₂ + 4Li⁺ + 4e⁻, is as high as 45% since 1.8Li⁺ is reversible per reaction formula.¹⁸ The anode also provides a relatively high capacity of 615.5 mA h g⁻¹ at 1 A g⁻¹ even after 200 charge/discharge cycles. This synthesis process is thus an effective pathway to prepare high quality SnS₂/RGO nanocomposites with a large reversible capacity.

Scheme 1 Schematic illustration of the formation of self-assembled SnS₂/RGO nanocomposite in solvothermal process.

2. Experimental

2.1. Preparation of SnS₂/RGO nanocomposites

All chemical reagents were analytical grade and used without further purification. SnCl₄·5H₂O and thiourea were purchased from Aladdin Industrial Corporation. GO was made by the modified Hummers method, exfoliated using a high pressure homogenizer, and annealed at 300 °C for 3 h under Ar atmosphere, yielding reduced graphene oxide (RGO).

In the typical experiment, SnS₂/RGO nanocomposites were synthesized by a solvothermal method. 0.1753 g SnCl₄·5H₂O and 0.0952 g thiourea were dissolved in 30 ml of ethylene glycol by ultrasound to give a transparent solution. 0.0351 g RGO was added into the mixture and then transferred into a Teflon-lined autoclave (50 ml) in an oven at 180 °C for 24 h. After cooling to room temperature, the precipitate was collected from the solution through centrifugal filtration, washed several times using distilled water to remove the organic residues, and dried at 60 °C for 6 h.

2.2. Materials characterizations

Field emission scanning electron microscopy (FESEM; JSM-7000F) was used to determine the morphology of the samples. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL model JEM2100 instrument at an accelerating voltage of 200 kV. The crystal phase properties of the samples were analyzed with a Bruker D8 Advance X-ray diffractometer (XRD) using Ni-filtered Cu Kα radiation at 40 kV and 40 mA and 2θ ranging from 10° to 90° with a scan rate of 0.02° per second. The energy dispersive X-ray spectrum (EDX) was used to further confirm the chemical composition. Fourier transform infrared spectroscopy (FTIR) was performed using a NEXUS870 spectrometer from 2000 cm⁻¹ to 200 cm⁻¹. Raman spectra were obtained using a Raman spectrometer (JY T64000) excited by the 514.5 nm line of an Ar⁺ laser under 100 μW. X-ray photoelectron spectroscopy (XPS) analysis (PHI5000 Versaprobe) was used to determine the chemical composition of the products. Thermogravimetric (TG) experiments were conducted on a simultaneous thermal analyzer (DTA-TG, NEFZSCH STA 499F3) in an Ar/O₂ (92 : 8) atmosphere from 30 °C to 850 °C at a heating rate of 10 °C min⁻¹. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method with the nitrogen adsorption/desorption isotherm.

2.3. Electrochemical measurements

For lithium ion battery measurements, a slurry of active materials mixed with carbon black and polyvinylidene fluoride (PVDF) was prepared in a weight ratio of 80 : 10 : 10, to form homogeneous slurry in N-methyl-pyrrolidone (NMP). The slurry was then coated as a thin film of about 50 μm thickness on a copper foil and dried in vacuum at 80 °C for 12 h. A certain pressure was applied to press the electrodes to enhance the contact between the active materials and the conductive carbon. The coin cell assemblies were assembled at room temperature in a glove box under argon. The mass loading of the active material on working anode electrode was about 0.21 mg. Cyclic voltammograms and galvanostatic charge/discharge cycling tests were performed at current rates of 0.1 A g⁻¹ in the voltage range of 0.01 and 3 V (versus Li⁺/Li). The discharge capacity of SnS₂, RGO and SnS₂/RGO nanocomposites electrodes were probed at current density of 0.1, 0.2, 0.4 and 0.8 A g⁻¹ respectively. The 200-cycles discharge capacity of RGO and SnS₂/RGO nanocomposites electrodes were probed at a current density of 1 A g⁻¹. The first five cyclic voltammograms (CV) were performed over a voltage range of 0.01–3.00 V at a scanning rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements of the fresh cell and after-cycling electrodes were obtained by applying a sine wave with an amplitude of 5.0 mV over the frequency range of 1000 kHz to 0.01 Hz.

3. Results and discussion

The reversible SnS₂/RGO nanocomposites were synthesized via a facile and effective solvothermal method hinging upon the advantages of ethylene glycol. The possible chemical reactions are proposed in Scheme 1. At the early stage, the synthesis approach is based on the self-assembly of a chelated Sn precursor, formed by the reaction between SnCl₄·5H₂O and ethylene glycol. Subsequently, it is hypothesized that the chelation complex will be adsorbed on the RGO nanosheet surface to form the chelation/RGO hybrid due to the strong affinity between the functional groups of the chelation, such as hydroxyl, carboxyl or epoxy groups, and RGO by condensation reactions.^19,25–27 Thiourea will then be hydrolyzed to release H₂S as a sulfide source that will combine with the chelation/RGO hybrid in the solution.²⁸ Finally, SnS₂ nanoparticles will be simultaneously attracted and captured into the RGO nanosheets to form network architectures during the solvothermal process.^19,29

The morphologies of SnS₂/RGO nanocomposites were observed by FESEM as shown in Fig. 1a and b. Fig. 1a shows the 3D sheet-on-sheet multilayer nanostructure formed by 2D sheet-like building units self-assembled to into porous structures using the solvothermal reaction, which can lead to high surface area density and plenty of channels for Li⁺ transportation.¹⁷ The high magnification FESEM image (Fig. 1b) clearly shows SnS₂ nanoparticles homogeneously distributed on the RGO nanosheets. As shown in ESI Fig. S1,† the obtained product exhibits a porous structure with a relatively high BET specific surface area of 111.8 m² g⁻¹ and pore volume of 0.2023 cm³ g⁻¹ (p/p₀ = 0.95).^10,27 The surface area of the SnS₂/RGO nanocomposite is largely improved due to the porous lamellar structure, which will enhance the electrochemical performance. The TEM image reveals that SnS₂ nanoparticles are successfully distributed on the RGO nanosheets (Fig. 1c). It has also been observed that the solvent and mass ratio of RGO/SnCl₄·5H₂O prescribed during synthesis are vital factors in controlling the morphology of SnS₂/RGO compounds (Fig. S2 and S3†). The HRTEM image (Fig. 1d), taken from a selected area in Fig. 1c exhibits parallel fringes with interplanar distance of 0.28 nm and 0.34 nm, which are in accordance with the d-spacing values of the SnS₂ (101) and RGO (002) crystalline planes respectively.^7,10,22

Fig. 1 (a) Low and (b) high magnification FESEM images of SnS₂/RGO nanocomposites; (c) TEM image of SnS₂/RGO nanocomposites; (d) its HRTEM image.

The crystalline phases from the prepared samples of SnS₂ nanoparticles and SnS₂/RGO nanocomposites were probed using X-ray diffraction (XRD). Fig. 2a shows four representative diffraction peaks of (100), (101), (110) and (111) from pristine SnS₂, confirming the well-defined crystallinity of the as-prepared SnS₂/RGO nanocomposites. All of the diffraction peaks are in agreement with the standard patterns of hexagonal SnS₂ (JCPDS 23-0667).² Diffraction peak positions of SnS₂/RGO nanocomposites that are contributed by SnS₂ are similar to that of SnS₂, with higher diffraction peaks intensities. There is a sharp diffraction peak at about 2θ = 25.8°, which can be attributed to the (002) plane of reduced graphene oxide (RGO). This confirms the presence of RGO during solvothermal treatment as previously reported.^2,3,24,30 The energy dispersive X-ray spectrum (EDX) analysis (as shown in Fig. S4†) further confirm the chemical composition, where the C, S and Sn elements are detected, and the atomic ratio of Sn : S is estimated to be about 1 : 2, indicating the highly efficient combination of SnS₂ nanoparticles and RGO nanosheet.^5,22

Fig. 2 (a) XRD pattern of SnS₂ (black) and SnS₂/RGO nanocomposites (red). (b) Raman spectra and (c) FT-IR of RGO (black) and SnS₂/RGO nanocomposites (red) synthesized, (d) TG-DTA curves of SnS₂/RGO nanocomposites.

The chemical structure was also investigated by Raman and FTIR spectroscopy, which are shown in Fig. 2b and c respectively. The Raman spectra of RGO and SnS₂/RGO nanocomposites are shown in Fig. 2b. The peak at about 1350 cm⁻¹ (D band) is related to the defects and disorder in the hexagonal graphitic layer, while the peak at about 1588 cm⁻¹ (G band), corresponding to the E_2g mode of graphite, is related to the vibration of the sp²-bonded carbon atoms in a 2-dimensional hexagonal lattice.^22,31,32 The intensity ratio of the D band to the G band (I_D/I_G), which is related to the density of defects in graphene-based materials, is calculated to be 1.34 and 1.30 for RGO and SnS₂/RGO nanocomposites respectively.²¹ The I_D/I_G of SnS₂/RGO is slightly lower than RGO, indicating that less defects and disorder have been doped into SnS₂/RGO, which may be beneficial to the lithium-ion storage capacity.^19,23 The fundamental peak observed at around 311 cm⁻¹ in inset of Fig. 2b (the enlarged peaks of the black rectangle area) for the SnS₂/RGO spectrum corresponds to the A_1g mode of SnS₂, which, according to group theory analysis conducted by previous studies, confirms the formation of SnS₂/RGO nanocomposites.^20,21,31,32

The FTIR spectra of RGO and SnS₂/RGO (Fig. 2c) are similar and contain two peaks at 3429 cm⁻¹ and 1112 cm⁻¹. The absorption peak at 3429 cm⁻¹ can be assigned to the O–H stretching vibration of hydroxyl groups in RGO and water molecules. The other peak at 1112 cm⁻¹ is related to the presence of C–O functional groups on the surface of RGO nanosheets. The weak absorption features in the blue region from ∼1400 to ∼1600 cm⁻¹ correspond to the carboxylic groups (C=O) stretching of various functional groups.^6,23 The significant difference between RGO and SnS₂/RGO is that the peak at ∼633 cm⁻¹ that corresponds to Sn–S bond stretching can be only observed from the spectrum of SnS₂/RGO nanocomposites.^6,33 The infrared difference spectrum (inset of Fig. 2c) also indicates the presence of Sn–S. The presence of oxygen-containing groups is important as they may facilitate the absorption and dispersion of SnS₂ nanoparticles on the RGO surface, as illustrated in Scheme 1.^22,34

The content of SnS₂ in the SnS₂/RGO nanocomposites plays an important role in their lithium storage performance, so TG experiments were conducted to estimate the amount of SnS₂ nanoparticles dispersed on the RGO nanosheets surface, as shown in Fig. 2d.³⁵ The TG-DTA profiles of SnS₂/RGO nanocomposites show three distinct regions of weight loss. The initial weight loss up to 200 °C (∼0.67%) is ascribed to the evaporation of absorbed water. The weight loss from 200 to 700 °C is mainly due to decomposition of the residual oxygen-containing groups and oxidation of SnS₂ into SnO₂, resulting in a total weight loss of about 48.25%.³⁶ The amount of SnS₂ is calculated from the residual SnO₂, as the substitution of higher the molecular weight S atoms by O atoms can cause weight loss. Accordingly, the weight fraction of SnS₂ in the SnS₂/RGO nanocomposites can be calculated to be 62.74 wt%, indicating high loading and uniform deposition on the RGO nanosheets.^5,34 The large loading quantity of SnS₂ nanoparticles on RGO nanosheets may be a crucial property for high performance lithium storage.

The X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical composition of SnS₂/RGO nanocomposites, as shown in Fig. 3. Sn, S, C and O were detected (Fig. 3a) and the binding nature of C 1s, Sn 3d and S 2p were investigated in detail.³⁶ The C 1s (284.6 eV) spectra can be fitted into three peaks corresponding to the presence of C–C (284.4 eV), C–O (285.0 eV) and C=O (286.7 eV) respectively (Fig. 3b).^1,28,31,37 These oxygen-containing functional groups on the surface of RGO possibly play an important role in fixing and dispersing the SnS₂ nanoparticles.² The binding energies of Sn 3d_5/2 and Sn 3d_3/2 can be observed at about 487.6 eV and 496.0 eV and no obvious Sn²⁺ peak (at around 485.8 eV) is detected (Fig. 3c). This is consistent with the reference values for the SnS₂ nanocrystal.²⁸ The S 2p peak can be fitted with a pair of binding energies at 163.9 eV and 164.8 eV, assigned to S 2p_3/2 and S 2p_1/2 respectively (Fig. 3d).^31,36,37 These results are consistent with the FTIR and Raman spectra.

Fig. 3 (a) Full XPS spectrum of the SnS₂/RGO nanocomposites; (b) XPS spectrum of curve fitting and splitting peaks from C 1s; (c) high-resolution XPS spectra of the Sn 3d, (d) XPS spectrum of curve fitting and splitting peaks from S 2p.

The lithium ion storage behavior is characterized by the first five cyclic voltammograms (CV) plots for bare SnS₂ and SnS₂/RGO nanocomposites (Fig. 4a and b). Two reduction peaks can be observed at ∼1.1 V (Fig. 4a) and ∼0.7 V (Fig. 4b) respectively for the first cycle but disappeared in the following cycles, corresponding to the decomposition of SnS₂ into metallic Sn and Li₂S and the formation of solid electrolyte interface (SEI) film as represented by reaction (1).^3,38 These processes are generally believed to be irreversible, resulting in large irreversible capacity in the first charge/discharge cycle. The characteristic cathodic/anodic pair observed at around 0.1 V/0.6 V in the first scan and in a similar potential range in the following cycles can be attributed to the alloying/dealloying reversible reactions of Li ion and Sn metal described by reaction (2).^4,14,17 The reduction peaks at ∼1.4 V from SnS₂ can be attributed to lithium intercalation into the SnS₂ layers without causing phase decomposition and the two oxidation peaks at ∼1.2 V and ∼1.8 V (Fig. 4a and b) can be attributed to the deintercalation of lithium ions into the SnS₂ layers without phase decomposition.^15,32 It is worth noting that the CV plots of SnS₂/RGO after the first cycle are almost identical, indicating the good cycling stability of the composite provided by the conductive effect of RGO and ultrafine SnS₂ nanoparticles. In contrast, the peak intensity of bare SnS₂ decreased gradually with cycling, resulting from the degradation of electrode kinetics.² For SnS₂/RGO nanocomposites, a small oxidation peak at around ∼2.3 V may be related to the partial decomposition of Li₂S and the two reduction peaks at ∼1.0 V and ∼1.7 V may be attributed to the supply of Li₂S to sustain the constant rate of Li₂S decomposition in the subsequent scans, which contributes to the rather high capacity as compared to bare SnS₂.^2,18 According to recent relevant publications, the decomposition of Li₂S and recovery of SnS₂ may only be partially completed.^2,18 The introduction of RGO nanosheets may lead to an increase in the reversible capacity. Graphene sheets act as buffer layers for the volume expansion not only to supply conductive channels for Li⁺ in the electrolyte, but also to allow for the growth and uniform anchoring of nanoparticles by preventing agglomeration at high loading.^14,32 If Li₂S can be decomposed completely according to the new storage mechanism:^2,18

SnS₂ + 8.4Li⁺ + 8.4e⁻ ↔ Li_4.4Sn + 2Li₂S (3)

Fig. 4 Cyclic voltammetry curves of (a) SnS₂ and (b) SnS₂/RGO at a scan rate of 0.1 mV s⁻¹ in the potential range of 0.01–3.00 V, (c) rate capabilities of RGO anode, SnS₂ anode and SnS₂/RGO anode; inset: discharge/charge voltage profiles of at current density of 0.1, 0.2, 0.4, and 0.8 A g⁻¹, (d) Nyquist plots of the SnS₂ and SnS₂/RGO after cycling electrodes obtained by applying a sine wave with an amplitude of 5.0 mV over the frequency range of 1000 kHz to 0.01 Hz, (e) discharge capabilities profiles of RGO and SnS₂/RGO electrodes at a big current of 1 A g⁻¹.

The theoretical capacity of SnS₂ based on the theoretical maximum Li/Sn stoichiometry of 8.4 is 1231 mA h g⁻¹.¹⁹ The redox peaks of CV plots are well matched to the discharge/charge voltage profiles plateaus of SnS₂ and SnS₂/RGO anode at a current of 0.1 A g⁻¹ performed.

Fig. S5a and 5b† shows a higher initial capacity of 2190.5 mA h g⁻¹ for SnS₂/RGO anode as compared to the initial capacity of 1382.4 mA h g⁻¹ for SnS₂, achieved at a current of 0.1 A g⁻¹.¹⁴ Even after 30 cycles, the discharge capacity value of about 939.9 mA h g⁻¹ of the SnS₂/RGO architecture is nearly twice as large as the value SnS₂ (564.6 mA h g⁻¹), which can be ascribed to a highly conductive network of RGO nanosheets.^2,4 SnS₂/RGO delivered a charge capacity of 914.6 mA h g⁻¹ after 30 cycles and reversibly stored and retrieved 6.2Li⁺ per Sn atom. Therefore, the reversibility of the conversion reaction (Sn + 2Li₂S → SnS₂ + 4Li⁺ + 4e⁻) is as high as 45%.¹⁸ On the other hand, only 3.8Li⁺ per Sn atom were reversibly stored and retrieved after 30 cycles for SnS₂, according to the charge capacity of 555.2 mA h g⁻¹. This can be attributed totally to the electrochemically inactive Li₂S based on the irreversible reaction (1).² This provides a reasonable explanation to the higher reversible capacity of SnS₂/RGO compared to the corresponding theoretical value. This result proves that this synthesis technique can be effectively used to achieve large loading quantity of nanoparticles on RGO nanosheets and a greater reversible capacity. We also speculate that the presence of the Sn nanoparticles possesses the catalytic effect for the reversible decomposition and formation of Li₂S.¹⁸Fig. 4c shows the rate performances of SnS₂, RGO and SnS₂/RGO anode at various current densities of 0.1 A g⁻¹, 0.2 A g⁻¹, 0.4 A g⁻¹ and 0.8 A g⁻¹, which demonstrates the good high-rate performance of the SnS₂/RGO anode. Compared to bare SnS₂ and RGO, the SnS₂/RGO anode delivered an average rate capacity of 939.0 mA h g⁻¹, 808.3 mA h g⁻¹, 740.1 mA h g⁻¹ and 650.5 mA h g⁻¹ respectively.³⁸ These values are much higher than the general theoretical capacity of the individual materials, graphite (372 mA h g⁻¹) and SnS₂ (645 mA h g⁻¹ based on 4.4Li/Sn) and the discharge capacities after the same number of cycles, which may be partly attributed to the advantageous synergistic effect of SnS₂ nanoparticles and RGO nanosheets.^1,2,18 The presence of RGO acts a good buffering matrix to buffer large volume expansion during the formation of Li–Sn alloys, inhibit the aggregation of nanoparticles, increase contact area of active materials/electrolyte, shorten the path length of Li⁺ transport and serve more conductive channels to accommodate Li-ion.^34,36,39 SnS₂ nanoparticles can provide high reversible capacities and prevent the fold of RGO nanosheets, which is favorite for superior electrical stability.³⁹ The synergistic effect between the two components can be seen during lithium ion insertion and extraction (ESI Scheme S1†). When the current density was set back to 0.1 A g⁻¹ again, the rate capacities were, for SnS₂ and RGO, SnS₂/RGO anode respectively, 532.1 mA h g⁻¹, 437.6 mA h g⁻¹, and 1133.5 mA h g⁻¹ after 70 cycles. In the last 10 cycles, the slightly enhanced capacity may be attributed to possible interfacial lithium storage. The very good cycling stability and resilience of the electrode are fully indicated in such rate capability. It is believed that the extra amount of lithium ion storage capacity may be stored in the pores or microcavities between the self-assembled sheet-on-sheet packed porous network structure, resulting in more void space and plenty of channels for the Li⁺ diffusion.²²

Electrochemical impedance spectroscopy (EIS) measurements are also performed on bare SnS₂ and SnS₂/RGO to understand the reasons for the superior performance of SnS₂/RGO.⁷Fig. 4d presents the impedance plots of the two electrodes along with the equivalent circuit model. The depressed semicircle at middle and high frequency represents the internal resistance (R_e) of the battery, the resistance (R_f) and constant phase element (CPE₁) of SEI film, as well as the charge transfer resistance (R_ct) and constant phase element (CPE₂) of electrode/electrolyte interface.^5,9 It is clear that the size of semicircle of SnS₂/RGO is smaller than that of bare SnS₂, indicating a lower R_f and R_ct of SnS₂/RGO. The inclined line in the low frequency range is associated with the Warburg impedance (Z_w), corresponding to the lithium diffusion process within the bulk of the electrode materials.⁵ The fitted parameter for the charge transfer resistance R_ct of the SnS₂/RGO electrode is 60.43 Ω, which is much less than the corresponding value of 81.81 Ω for bare SnS₂.^5,9 The structure of the nanocomposite, with small SnS₂ nanoparticles uniformly adhered on RGO surface is beneficial for the fast transportation of electrons and Li⁺. This is crucial in enhancing the conductivity performance during electrochemical lithium insertion/extraction. Compared to the other SnS₂/RGO hybrids, the performance of our prepared SnS₂/RGO nanocomposites is found to be much better than those of most reported anode materials for Li-ion batteries shown in Table 1.^{21–23,26,30,35}

Table 1 Specific capacities of tin sulfide based materials as anode materials for LIBs

Material	Current rate (mA g^−1)	Specific capacity (mA h g^−1)	Ref.
a RGO is reduced graphene oxide. b G is graphene. c 1C = 645 mA g^−1. d GNS is graphene nanosheets. e C is carbon.
SnS2–RGO^a	200	654 (20th)	3
G–SnS2	50	650 (30th)	21
SnS2@RGO	120	619 (30th)	23
G–SnS2	200	351 (50th)	24
RGO–SnS2	0.1C	662 (10th)	30
SnS2@G^b	0.5C^c	542 (10th)	32
SnS2/GNS^d	0.1C	747 (10th)	36
SnS2/C^e	50	600 (50th)	38
SnS2/RGO	100	939 (30th)	This work

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The nanocomposite also exhibits excellent high-rate and long-term cycle performances at a large current rate of 1 A g⁻¹ for 200 cycles (Fig. 4e). Initial discharge capacity of SnS₂/RGO is 1609.8 mA h g⁻¹, which decreased to 615.5 mA h g⁻¹ after 200 cycles.^1,3,15 In comparison, the RGO electrode has a smaller discharge capacity of 175.8 mA h g⁻¹ after 200 cycles, as observed in Fig. 4e, which can be due to the electronic conductivity of unsupported SnS₂.²² The SnS₂/RGO electrodes are clearly superior compared to bare SnS₂ and RGO by comparison, indicating the ease of ion diffusion and ability to withstand volume changes during the Li⁺ insertion/extraction process. In addition, the Sn nanoparticles formed in reactions between Li⁺ with SnS₂ may lead to the additional capacity during subsequent cycling. It is hypothesized that the conversion reaction (the reversible decomposition and formation of Li₂S) can be catalyzed from the catalytic effect of metallic Sn, in which Sn NPs are held firmly between RGO nanosheets.¹⁸ This trend can also be found in previous reports.^15,18,26,40 TEM image in the presence of carbon black conducting agent and PVDF binder material of the cycled electrode is shown in Fig. S6a.† The original composite morphology can be basically retained after repetitive lithium ion insertion and extraction, which indicates the porous nanocomposite electrode can buffer large volume change.¹⁵ Fig. S6b† shows the XRD of SnS₂/RGO electrode after charge–discharge cycles, which exhibits Sn, Li–Sn alloys and Li₂S phase form after Li⁺ addition.²

4. Conclusions

In summary, in the study of the reversibility of the conversion reaction, SnS₂/RGO nanocomposites were successfully synthesized by a facile one-step solvothermal method using ethylene glycol as a chelant. The SnS₂ nanoparticles are well dispersed on RGO nanosheets surfaces due to the sophisticated design of the sheet-on-sheet packed porous structure. When used as anodes in lithium ion batteries, the obtained nanocomposites exhibited a higher reversible Li⁺ storage discharge capacity of 939.0 mA h g⁻¹ at 0.1 A g⁻¹ after 30 cycles, and more impressively, 615.5 mA h g⁻¹ at 1 A g⁻¹ even after 200 cycles. The enhanced electrochemical performance of the SnS₂/RGO hybrid anode will be important for the future development of high performance electrode materials.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (973 Program: 2013CB932903), the National Science Foundations of China (No. 61205057, No. 11574136), Qing Lan Project, the ‘1311 Talent Plan’ Foundation of Nanjing University of Posts and Telecommunications, Six talent peaks project in Jiangsu Province (JY-014), and Jiangsu Provincial Key R & D Program (Grant No. BE2015700).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03124b

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