Microwave-assisted ionothermal synthesis of SnSe_x nanodots: a facile precursor approach towards SnSe₂ nanodots/graphene nanocomposites

Abstract

Presented here is a facile approach towards spherical SnSe₂ nanodots/graphene nanocomposites via an ionic liquid media assembly process, which involved an easily scaled-up microwave-assisted ionothermal synthesis of SnSe_x nanodots (NDs) as precursors by the reaction of elementary tin and selenium in (Bmmim)Cl (Bmmim = 1-butyl-2,3-dimethyl imidazolium) followed by an assembly process under ambient conditions and subsequent thermal treatment. The regulation of the content of SnSe₂ NDs could be conveniently achieved by varying the ratio of ND precursor to graphene oxide (GO). As evidenced by TG-MS and FTIR analysis, the assembly process could be attributed to the electrostatic interaction between the anionic GO and the positively charged SnSe_x NDs, which are fixed in the IL cation layer around the NDs as a medium. This conclusion was further confirmed by the TEM micrographs, which showed a constant particle size in the precursor and in the nanocomposites after thermal treatment. Lithium storage characterizations showed that the capacity of the as-prepared nanocomposite remained at 659 mA h g⁻¹ after 30 cycles at a current density of 150 mA g⁻¹, which is 1.5-times better than the theoretical capacity of SnSe₂ (426 mA h g⁻¹) and superior to the capacities of the previously reported SnSe₂ nanoplate/graphene composite and many other tin selenide electrodes. Thus, the new approach represents a promising, simple, and scalable synthetic protocol for the fabrication of lamellar metal dichalcogenide (LMD) NDs/graphene nanocomposites.

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Introduction

Due to the low cost, high surface area, high conductivity and high chemical inertness, graphene and its derivatives have attracted particular interest as functional materials.^1–8 They can be compounded with various nanomaterials to form graphene-based nanocomposites, which have been widely used in energy conversion and storage, catalysis, environmental, medical and electronic applications.^9–15

The control of the size, morphology and dispersion of nanomaterials on graphene is crucial for developing high-performance graphene-based nanocomposite materials. For instance, the small particle size of electrode materials for Li-ion batteries can endow high surface area to buffer the volume change and shorten the diffusion length for Li ions, thus improving the cycle stability and rate capability.^16,17 Therefore, nanomaterials with certain morphologies, e.g. nanodots (NDs), are highly desirable in graphene-based nanocomposites. To date, a number of metal oxide NDs/graphene nanocomposites have been obtained.^17–19 In contrast, the lamellar metal dichalcogenides (LMDs), an important class of lithium storage materials, are strongly inclined to grow on graphene two-dimensionally (2D) as nanoplates or nanoshells due to the lamellar nature of LMDs and the graphene support.^20–24 Thus far, the synthesis of LMDs NDs/graphene nanocomposites has proven challenging.²⁵

Ionic liquids (ILs) have emerged as a type of functional material and green solvent.^26–31 The unique physical properties of ILs, such as high thermal stability, high solubility, low vapour pressure, low toxicity and self-template effect, make them suitable for inorganic synthesis.^30,32 It has been demonstrated that ILs can serve as alternative solvents in the synthesis of metal chalcogenides.^33–37 On the other hand, due to the high microwave absorbing property of ILs, the microwave-assisted ionothermal process has become a promising route to acquiring various metal chalcogenide nanomaterials.^33,38,39 In our previous work, we found that the ionothermal treatment of a mixture of Sn and Se powders in 1,2,3-trialkylimidazolium-based ILs could result in a homogeneous ionic gel, from which the nanoparticles of new selenidostannate phases could be obtained.³⁴ In addition, the high affinity of ILs on nanocarbon and their mediator role in the assembly of nanocomposites has recently received considerable attention.^40–42 Therefore, we hoped to develop an IL-media assembly approach towards LMD NDs/graphene nanocomposites utilizing the microwave-assisted ionothermal synthesized LMD NDs as precursors.

Herein, by choosing SnSe₂, a typical LMD, as the model compound, a facile approach towards spherical SnSe₂ NDs/reduced graphene oxide nanocomposites (denoted as SnSe₂ NDs@rGO) was developed. The approach involved the microwave-assisted ionothermal synthesis of SnSe_x NDs@Bmmim precursors (Bmmim = 1-butyl-2,3-dimethyl imidazolium) in the first step followed by an IL-media assembly process and thermal treatment. The lithium storage characterizations showed that the as-prepared SnSe₂ NDs@rGO nanocomposites had a good lithium ion storage performance; the optimal SnSe₂ NDs@rGO nanocomposite exhibited a reversible capacity of 717 mA h g⁻¹, and the capacity was retained at 659 mA h g⁻¹ after 30 cycles at a current density of 150 mA g⁻¹.

Experimental

Materials and physical measurements

Tin (99.5%) powder was purchased from Sinopharm Chemical Reagent, selenium (analytical grade) powder was purchased from Tianjin Yingda Rare Chemical Reagent, and (Bmmim)Cl (99%) was purchased from Lanzhou Greenchem ILS, LICP. CAS. China. GO powder that was synthesized by a common Hummers' method was provided by XFNANO Materials Tech Co., Ltd. All of the chemicals were used without further purification. Field-emission scanning electron microscopy (FESEM) images were acquired on a JSM-6700F field-emission scanning electron microscope. Transmission electron microscopy (TEM) images were acquired on a JEM-2010 transmission electron microscope. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Mini Flex II diffractometer using CuKα radiation. Elemental analysis was performed on a German Elementary Vario EL III instrument. Fourier-transform infrared (FTIR) spectroscopy was carried out on a Vertex 70 FTIR spectrophotometer using KBr pellets within the range of 4000–400 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi XPS spectrometer. Thermogravimetric analysis (TGA) was carried out at a heating rate of 5 °C min⁻¹ in a N₂ atmosphere from 30 °C to 800 °C on a NETZSCH STA 449F3 unit under N₂ atmosphere. Thermal analysis-mass spectroscopy (TG-MS) was carried out at a heating rate of 5 °C min⁻¹ in N₂ atmosphere from 30 °C to 800 °C on a STA449C-QMS403C thermal analysis-quadrupole mass spectrometer.

Preparation of SnSe_x NDs precursor

The precursor SnSe_x NDs with diameters of 5 nm were prepared through a microwave-assisted ionothermal process. The microwave experiments were carried out on a microwave reactor with a maximum power of 300 W (Biotage Initiator™ EXP). Typically, 0.238 g of tin powder (2 mmol) and 0.348 g selenium powder (4.4 mmol) were mixed with 2 g of (Bmmim)Cl (10.4 mmol, Bmmim = 1-butyl-2,3-dimethylimidazolium) in a 10 mL Biotage microwave vial. The vial was sealed and heated (100 °C) in a water bath to form a mash-like mixture. The mixture was then heated under microwave using a controlled program (typically, the mixture was heated at 120 °C for 5 minutes followed by 180 °C for 55 minutes and then cooled to room temperature) with magnetic stirring, which resulted in a transparent dark-red solution. The maximum heating power was set to 100 W. During the cooling process, the viscosity of the product increased significantly. Finally, a dark-red gel containing crystalline spherical SnSe_x NDs was formed.

Assembly of SnSe_x NDs with GO

The above SnSe_x NDs precursor was dissolved in 15 mL 1-N-methyl-2-pyrrolidinone (NMP) under stirring for 5 minutes, and the resultant solution was kept at 4 °C for 5 hours to allow the precipitation of the excess (Bmmim)Cl. After centrifugation, a clear orange solution was obtained for further characterization.

For the assembly of NDs with GO, 200 mg GO was dispersed in 5 mL deionized water (DI water) and sonicated for 30 minutes. Then, 10 mL of absolute ethanol and 10 mL NMP were added, and the mixture was sonicated for another 30 minutes. The clear NDs precursor/NMP solution was added dropwise into the GO solution under vigorous stirring and kept stirring for 30 minutes under ambient conditions. Then, 50 mL of absolute ethanol was added followed by the dropwise addition of 20 mL DI water. The solution was stirred for 30 minutes, and the nanocomposites of [Bmmim]⁺ cation-wrapped NDs encapsulated in GO (denoted as SnSe_x NDs@Bmmim@GO) were harvested by suction filtration and washed with water and ethanol several times. The products were then dried at 60 °C under vacuum for 5 hours. In order to obtain well-crystalline SnSe₂ NDs@rGO nanocomposites, the as-synthesized products were annealed under N₂ at 300 °C for 2 hours at a heating rate of 2 °C min⁻¹.

Three composites denoted as NG-1, NG-2 and NG-3 were synthesized by 200 mg GO composited with different amounts of NDs precursor/NMP solutions (3, 7, and 14 mL, respectively). For the control experiments, the annealed GO (in absence of NDs precursor) and annealed NDs precursors were also prepared using similar routes.

Electrochemical measurements

The electrochemical tests were performed using CR2025 coin-type test cells. To fabricate the working electrodes, 80 wt% active material, 10 wt% conductivity agent (TIMCAL SUPER C45 Carbon Black), and 10 wt% polyvinylidene fluoride binder (PVDF) were mixed with NMP and then pasted on Cu foil. The electrodes were dried at 60 °C for 3 hours followed by 120 °C for 15 hours in a vacuum oven. Cells were assembled in an Ar-filled glove box in which the concentrations of moisture and oxygen were below 1 ppm. Pure lithium foil was used as both the counter and reference electrodes. The electrolyte was 1 M LiPF₆ in ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by volume). A Celgard 2325 membrane was used as the separator. The galvanostatic discharge/charge cycles were carried out on a LAND 2001A system over a voltage range of 0.05 to 3.00 V at 30 °C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a CHI660E Electrochemical Workstation. The specific capacities in this article were calculated based on the overall masses of the nanocomposites.

Results and discussion

As shown in Scheme 1 and the ESI,† the overall synthetic procedure and assembly of SnSe₂ NDs@rGO nanocomposite involves three steps: (1) the preparation of SnSe_x NDs@Bmmim precursor under microwave-assisted ionothermal conditions; (2) the subsequent assembly process; and (3) post thermal treatment by annealing at 300 °C for 2 h. As confirmed by TGA-MS, FTIR and XPS analyses, the assembly of SnSe_x NDs with GO can be attributed to the electrostatic interactions between the anionic GO and the positively charged SnSe_x NDs, which are fixed in the IL cation layer around the NDs as a medium.^18,40,43,44 Three nanocomposites, denoted NG-1, NG-2 and NG-3, were synthesized with different ratios of SnSe_x NDs precursor to GO, as summarized in Table 1.

Scheme 1 The synthesis and assembly process of SnSe₂ NDs@rGO nanocomposites.

Table 1 Ratios of SnSe_x NDs precursor to GO for the three nanocomposites

	SnSex NDs precursor^a (mL)	GO (mg)	Elemental analysis of carbon content (%)
a 1-N-Methyl-2-pyrrolidinone (NMP) solution.
NG-1	3	200	52.67
NG-2	7	200	41.20
NG-3	14	200	25.43

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Fig. 1a and b show the typical heating profiles of the program-controlled microwave-assisted ionothermal reaction and the photograph of an as-prepared precursor, respectively. After a very fast microwave-assisted ionothermal reaction, the homogeneous gel-like NDs precursor was obtained. Notably, the pressure of the microwave reactor was constantly 0 bar over the reaction process, implying that the reaction will remain safe when scaled up (Fig. S1, ESI†). The NDs precursor was then dispersed into 1-N-methyl-2-pyrrolidinone (NMP) to remove the precipitation of the excess ILs for further assembly (Fig. S2, ESI†).

Fig. 1 (a) Microwave heating profiles (temperature, power, and pressure) for a typical synthesis of NDs precursor in [Bmmim]Cl. (b) The as-prepared NDs precursor.

The as-prepared SnSe_x NDs@Bmmim@GO composites were characterized by TG-MS analysis, PXRD and FTIR. As shown in Fig. S3,† the TG-MS profiles contained three weight-loss steps, clearly indicating the evaporation of adsorbed solvent, thermal decomposition of [Bmmim]⁺ and the phase transformation from SnSe₂ to SnSe during the heating process. The PXRD pattern plotted in Fig. S4† showed no characteristic peaks of SnSe₂, probably due to the formation of new selenidostannate phases.³⁴ As shown in Fig. 2 (middle), the FTIR spectrum of the pristine nanocomposite exhibited a broad peak at 1647 cm⁻¹, which could be assigned to the mixed peak of the C=O stretching band as the typical amide I band in case of cyclic tertiary amides in NMP and the C–C stretching vibration band of [Bmmim]⁺.^45,46 Several other characteristic peaks at 1734, 1402 and 1059 cm⁻¹ were assigned to the C=O and single bound carboxyl (C–O) vibrations in the carboxylic acid or carbonyl moieties and C–O vibrations in the alkoxy groups of GO.^43,47,48 Compared to the FTIR spectrum of (Bmmim)Cl (Fig. 2, top), the peaks at around 2925 and 2868 cm⁻¹ were attributed to the two C–H vibrations of the –CH₃ and –CH₂ bands in [Bmmim]⁺. The C=C and C=N stretching, C–N stretching and in-plane C–H bending vibration of [Bmmim]⁺ could be found at 1498, 1300 and 1248 cm⁻¹.⁴⁹ The peaks at 829, 734 and 658 cm⁻¹ were attributed to the C–H and C–N out-of-plane bending vibrations of [Bmmim]⁺.^49,50 The broad peak at 3427 cm⁻¹ could be assigned to the stretching vibrations of hydroxyl groups.

Fig. 2 FTIR spectra of (Bmmim)Cl (top), the pristine SnSe_x NDs@Bmmim@GO nanocomposite (middle) and the annealed product (NG-3, bottom).

The annealed samples (e.g., NG-3) were also characterized by PXRD and FTIR. As illustrated in Fig. S4a,† all the peaks can be readily indexed to the hexagonal SnSe₂ phase (JCPDS card no. 89-3197), indicating high phase purity. As for the FTIR spectra of NG-3 (Fig. 2, bottom), the characteristic peaks of [Bmmim]⁺ disappeared, indicating the thermal decomposition and departure of [Bmmim]⁺ during the annealing process. Instead, three new peaks at 1699, 1558 and 1186 cm⁻¹ appeared. The peaks at 1186 cm⁻¹ and 1699 cm⁻¹ can be ascribed to the vibrational modes of basal plane epoxides (C–O–C) and the C=O of the stable acid moieties.^48,51 The peak at 1558 cm⁻¹ may be from the skeletal vibrations of unoxidized graphitic domains, indicating the reduction of GO.^43,51,52

The FTIR spectra clearly indicated the existence and thermolysis of [Bmmim]⁺ cations, implying that the NDs were uniformly encapsulated in an IL cation matrix in the pristine nanocomposites. Thus, the assembly of SnSe_x NDs with GO could be attributed to the electrostatic interaction between the anionic GO and the positively charged SnSe_x NDs, which are fixed in the IL cation layer around the NDs as a medium.^{40,43,44,51,52} Consequently, the SnSe₂ NDs could be assembled on the surface of rGO after annealing. To further confirm the composition of the as-obtained products and study the annealing process, XPS analysis was conducted on the annealed products. Fig. 3 shows the XPS spectra of NG-3. As shown in Fig. 3, the C1s peak in XPS spectra appeared at 284.8 eV, which was in accordance with both chemically converted and thermally converted graphene,⁵³ indicating the reduction of GO. The small peak centred at 288.5 eV was attributed to the residual C=O group.^54,55 The 3d peaks of Sn and Se appeared at 495.7, 487.3 eV and 55.5 eV, which matched well with those of tin and selenium in the stoichiometric SnSe₂ phase in the material.^21,56

Fig. 3 XPS spectra of NG-3.

The NDs precursor was investigated by TEM and high-resolution (HR) TEM. As shown in Fig. 4a and b, the SnSe_x NDs in the precursor have regular spherical shapes with an average diameter of about 5 nm. The TEM and HRTEM images of annealed products (for example, NG-3) are shown in Fig. 4c and d, indicating that the NDs were uniformly distributed on the rGO substrate. The parallel lattice fringes of the NDs with interplanar distances of 0.33 nm (Fig. 4d, upper inset) were consistent with the (100) plane of hexagonal SnSe₂, indicating the formation of SnSe₂ NDs after annealing, which was also confirmed by the FFT analysis (Fig. 4d, lower inset). Importantly, the sizes of SnSe₂ NDs remained around 5 nm without growth after annealing. The elemental mapping of SnSe₂ NDs@rGO nanocomposites on carbon, selenium and tin further demonstrates the homogeneous and highly efficient combination of SnSe₂ NDs and rGO (Fig. 4e). The morphology and microstructure of the as-prepared SnSe₂ NDs@rGO nanocomposites were also investigated using FESEM. As seen in Fig. 4f and g, the as-prepared SnSe₂ NDs@rGO nanocomposite (NG-3) possesses a rough surface and a disordered three-dimensional (3D) network structure composed of flexible rGO lamellae and SnSe₂ NDs with interconnected pores ranging from several hundreds of nanometres to several micrometres in size. All the above-mentioned characterizations indicated the successful preparation of SnSe₂ NDs@rGO nanocomposites in which the GO lamellae served as the ESI,† and the SnSe₂ NDs were distributed uniformly on the surfaces of the rGO lamellae and homogeneously in the entire network.

Fig. 4 TEM (a) and HRTEM (b) images of SnSe_x NDs in NMP. The TEM (c) and HRTEM (d) images of annealed NG-3; the insets in (d) show the interplanar distances of SnSe₂ NDs (upper) and the FFT analysis (lower). Element mapping images (e), SEM images of the three-dimensional network structure (f) and the rough surface (g) for annealed NG-3.

In addition, by modifying the ratios of SnSe_x ND precursor to GO, the loading contents of SnSe₂ NDs on the rGO substrate could be adjusted. Fig. 5 shows the representative TEM and HRTEM micrographs of the three nanocomposites. On the basis of the micrographs shown in Fig. 5, the loading of SnSe₂ NDs on the rGO substrate increased as the ratio of SnSe_x ND precursor to GO increased. Consistent with this qualitative observation, the elemental analysis of the carbon content in the three composites quantitatively confirmed the results (see Table 1). Obviously, the assembly process we developed here was convenient for the regulation of ND content. Notably, although the loading of SnSe₂ NDs on the rGO substrate increased, the diameters of the NDs in the three nanocomposites were nearly the same, further verifying that the SnSe_x NDs were uniformly encapsulated in a matrix of [Bmmim]⁺ cations that separated the NDs during the annealing process.

Fig. 5 TEM and HRTEM images of NG-1 (a and b), NG-2 (c and d) and NG-3 (e and f).

The preparative methods of metal chalcogenide nanomaterial/graphene nanocomposites include both one-pot and step-by-step strategies. In the one-pot synthesis, the typical hydro/solvothermal procedures often suffer from the high solvent pressure and expensive or highly toxic reagents like alkyl phosphine, CS₂ and H₂S, seriously limiting their practical applications.²⁴ The method we developed here is a step-by-step approach. Through a one-pot program-controlled microwave-assisted ionothermal process, crystalline spherical SnSe_x NDs with diameters of approximately 5 nm were easily obtained using low-cost elementary substances as reactants and could be separated and used as precursors for further assembly. In the subsequent assembly process, the ILs not only stabilized the spherical SnSe_x NDs, but also induced the SnSe_x NDs to preferentially form composites with rGO. Also worth mentioning is that the microwave-assisted ionothermal reaction was carried out under negligible vapour pressure within an hour, and the assembly process proceeded under ambient conditions, which saves time, and could easily be scaled up. The excess ILs dissolved from the NMP solution in step (1) could also be recycled for further use. To the best of our knowledge, this is the first report of the preparation of LMDs NDs/graphene nanocomposites.

Although SnSe₂ is a congener of SnS₂, which has been widely investigated as a lithium storage material, similar studies on SnSe₂ are still quite limited.²¹ Herein, the electrochemical performances of the SnSe₂ NDs@rGO nanocomposites were evaluated by CV and galvanostatic charge–discharge cycling. The CV curves for the first three cycles of NG-3 at a scanning rate of 0.1 mV s⁻¹ are shown in Fig. 6a. In the CV curves, the peak at 0.97 V in the first cathodic sweep disappeared in the following cycles, which can be attributed to solvent decomposition and the formation of a solid electrolyte interface (SEI) layer on the electrode surface.⁵⁷ After the first cycle, the four cathodic peaks at 0.25, 1.37, 1.52 and 1.65 V in the cathodic scan and three anodic peaks at 0.54, 1.25, and 1.89 V in the anodic scan for the electrode should mainly result from the Li alloying/dealloying reactions with SnSe₂.^58,59 According to the literature, the lithiation processes of tin dichalcogenides (Q = S or Se) could be elucidated as follows:

SnQ₂ + xLi⁺ + xe⁻ ↔ Li_xSnQ₂ (1)

Li_xSnQ₂ + (y − x)Li⁺ + (y − x)e⁻ ↔ Li_ySnQ₂ (2)

Li_ySnQ₂ + (4 − y)Li⁺ + (4 − y)e⁻ ↔ Sn + 2Li₂Q (1 ≤ x ≤ y ≤ 2) (3)

SnQ₂ + 4Li⁺ + 4e⁻ ↔ Sn + 2Li₂Q (4)

Sn + 4.4Li⁺ + 4e⁻ ↔ Li_4.4Sn (5)

Fig. 6 Electrochemical performance of SnSe₂ NDs@rGO nanocomposites for lithium storage. (a) CV curves for the first three cycles of NG-3 at a scanning rate of 0.1 mV s⁻¹. (b) EIS spectra of NG-3 before and after fully charge/discharge cycles in CV characterization. (c) Cycling performance over 50 cycles at a current density of 150 mA g⁻¹ for the three SnSe₂ NDs@rGO electrodes (NG-1, NG-2 and NG-3), the annealed GO and annealed SnSe₂ from NDs precursor. The coulombic efficiency of the three samples was shown at the top. (d) Rate capabilities of NG-3.

Reactions (1)–(3) represent the lithium intercalation into the chalcogenide layers and the conversion reactions above 1.3 V, which could be summarized by reaction (4). Reaction (5) represents the alloying reaction of lithium below 0.5 V.^59,60 During the subsequent cycles, the CV curves of the SnSe₂ NDs@rGO composites displayed a constant integral area, indicating that the nanocomposite was capable of electrochemical lithium storage. The electrochemical impedance spectra of the NG-3 electrode recorded before and after the CV characterization are shown in Fig. 6b. The semicircle diameters of the NG-3 electrodes decrease after the CV characterization, suggesting that after full charging/discharging cycles, the NG-3 possessed a higher electrical conductivity and a faster charge–transfer reaction for Li-ion insertion and extraction.⁶¹ Similar phenomena were also found in metal oxide and silicon anodes,^62,63 which might originate from the physical restructuring of the SEI film.⁶⁴

The cycling performances and rate capabilities of the nanocomposites are shown in Fig. 6c and d, respectively. As shown in Fig. 6c, the first discharge capacity of the as-prepared SnSe₂ NDs@rGO nanocomposites at a current density of 150 mA g⁻¹ could surpass 800 mA h g⁻¹, and the discharge capacity could be retained at 659 mA h g⁻¹ after 30 cycles, which is much higher than its theoretical value of 426 mA h g⁻¹. The capacity of the SnSe₂ NDs@rGO nanocomposites increased with increasing ND content; however, the cycling performance first decreased and then increased with increasing ND content. The differences among these three samples implied that the cycling stability of graphene-based nanocomposites is related to the dispersity of nanoparticles and graphene.⁶⁵ For comparison, the annealed NDs precursor and annealed GO cycled at a current density of 150 mA g⁻¹ are also shown in Fig. 6c. The cycling capacity of the annealed NDs precursor decayed rapidly to about 50 mA h g⁻¹ over several cycles. The same phenomenon occurred in simplex-annealed GO (about 200 mA h g⁻¹), which further confirms that the simplex SnSe₂ is unstable during the lithiation/delithiation process, and GO does not contribute considerably to the capacity of the integrated electrode. This illustrates that the nanocomposites of SnSe₂ NDs and rGO provide the majority of the capacity and stability of the integrated anode.

Fig. 6d shows the rate performances of the NG-3 nanocomposite at various current densities ranging from 50 to 5000 mA g⁻¹. These curves exhibited similar shapes with only a small over potential and high capacities even at large current densities. As shown in Fig. 6b, the specific capacities of NG-3 were 796, 668, 534, 435, 323, 244 and 162 mA h g⁻¹ when cycled at 50, 150, 500, 1000, 2000, 3000 and 5000 mA g⁻¹, respectively. Even at a current density of 1000 mA g⁻¹, NG-3 still delivered a capacity of 435 mA h g⁻¹, which was higher than the theoretical capacity (426.3 mA h g⁻¹) of the SnSe₂ anode. When cycled back to 50 mA g⁻¹, a capacity of 729 mA h g⁻¹ could be restored and kept stable, indicating the good stability of the NG-3 nanocomposite. To our knowledge, the only example of a SnSe₂/graphene composite for application in lithium-ion batteries is an SnSe₂ nanoplate/graphene composite,²¹ which could only retain a reversible discharge capacity of 470–640 mA h g⁻¹ at a current density of 40 mA g⁻¹ after 30 cycles (summarized in Table 2). Compared to the low current density and cycling capacity of the SnSe₂ nanoplate/graphene composite, the SnSe₂ NDs@rGO nanocomposites in this work exhibit an improved performance due to the good dispersion and high surface area.

Table 2 Comparison of the Li-ion storage performance of NG-3 to those of other tin selenide electrodes^a

Composition	Id (mA g^−1)	Num	Ctheo (mA h g^−1)	Cexp (mA h g^−1)	Improvement (times)	Ref.
a Id = current density. Num = cycle number. Ctheo = theoretical capacity. Cexp = experimental capacity.b Reported in this work.
SnSe2 nanoplate/rGO	40	30	426	470–640	1.10–1.50	21
NG-3	150	30	426	659	1.55	^b
SnSe film	130	40	596	400	1.06	58
SnSe/carbon fibre	200	80	596	676	1.13	66
SnSe nanocrystals	60	30	596	<200	<0.34	67

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Conclusions

In summary, we demonstrated a novel step-by-step method for acquiring SnSe₂ NDs@rGO nanocomposites under ambient conditions. When evaluated as an anode material for lithium-ion batteries, these SnSe₂ NDs@rGO nanocomposites show significantly enhanced electrochemical performance with high capacity, excellent cycling stability and good rate capability. This method provides a simple and scalable synthetic approach for the fabrication of LMDs NDs/graphene nanocomposites.

Acknowledgements

This work was financially supported by the 973 program (no. 2014CB845603) and the NNSF of China (no. 21521061 and 21371001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Photographs, TGA, TG-MS and PXRD. See DOI: 10.1039/c5ra24500a

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