Synthesis and electrochemical properties of graphene-SnS₂ nanocomposites for lithium-ion batteries

Abstract

Graphene-SnS₂ nanocomposites were prepared via a solvothermal method with different loading of SnS₂. The nanostructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The XRD patterns revealed that hexagonal SnS₂ was obtained. SEM and TEM results indicated that SnS₂ particles distributed homogeneously on graphene sheets. The electrochemical properties of the samples as active anode materials for lithium-ion batteries were examined by constant current charge–discharge cycling. The composite with weight ratio between graphene and SnS₂ of 1:4 had the highest rate capability among all the samples and its reversible capacity after 50 cycles was 351 mAh/g, which was much higher than that of the pure SnS₂ (23 mAh/g). With graphene as conductive matrix, homogeneous distribution of SnS₂ nanoparticles can be ensured and volume changes of the nanoparticles during the charge and discharge processes can be accomodated effectively, which results in good electrochemical performance of the composites.

Introduction

Rechargeable lithium-ion batteries are currently the predominant power sources for portable electronic devices. To achieve improved electrochemical performance, new anode materials have been studied extensively. Among them, Li–Sn alloy compounds have attracted great interest due to their theoretical specific capacity up to 992 mAh/g [1]. SnS₂ was reported to be used as a novel anode material for lithium-ion batteries as it transforms to Li–Sn alloys during charge–discharge process [2, 3]. However, even though SnS₂ exhibits high initial discharge capacity, the large volume changes and loss of electrical contact during lithium intercalation and deintercalation result in capacity fading of the compound. Several approaches can be considered to enhance the capacity retention, such as reducing the particle size to nanoscale or dispersing the electroactive particles in a carbon matrix. It is believed that carbon matrix can buffer the volume changes and improve the electronic and ionic conductivities [4].

Graphene, which consists of two-dimensional sheets of covalently bonded carbon atoms, offers great advantages owing to its high surface area, good conductivity, and good mechanical properties [5, 6]. It is believed that the restacking of graphene nanosheets will be effectively prevented with nanosize particles deposited between them. Besides, the aggregation and pulverization of the active particle on cycling will be suppressed [7]. For these reasons, graphene has been studied as conductive matrix of lithium-ion battery electrode materials [8–10]. In this paper, we utilized functionalized graphene sheets (FGS) [11] as matrix to synthesize graphene-SnS₂ nanocomposites as lithium-ion battery anode materials via a solvothermal method. On this basis, we synthesized the nanocomposites with different loading of SnS₂. The obtained products exhibited enhanced cycle performance and lithium storage capacity compared with pure SnS₂ and FGS.

Experimental

Preparation of FGS

Graphite oxide (GO) was prepared by Hummers method [12]. To obtain FGS, certain amount of GO was thermally exfoliated at 300 °C for 5 min under air atmosphere and subsequently treated at 900 °C in N₂ for 3 h with a heating rate of 2 °C/min. The obtained sample was denoted as FGS.

Preparation of graphene-SnS₂ nanocomposites

All of the reactants and solvents were of analytical grade and were used without any further purification. To synthesize graphene-SnS₂ nanocomposite with theoretical weight ratio between FGS and SnS₂ of 1:1, FGS (100 mg) and SnCl₄·5H₂O (192 mg) were added in 100 mL of ethylene glycol. After stirring for 15 min, 164 mg of thioacetamide was dissolved in 10 mL of ethylene glycol and dropped into the mixture. After ultrasonication for 30 min, the mixture was transferred into a 150-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 190 °C for 12 h, and then allowed to cool to room temperature. The obtained black product was collected, washed with ethanol and distilled water several times, and dried under vacuum at 50 °C for 12 h. The obtained sample was denoted as GS1.

Parallel experiments were carried out to synthesize graphene-SnS₂ nanocomposites with theoretical weight ratio between FGS and SnS₂ of 1:2 and 1:4. The obtained samples were denoted as GS2 and GS3, respectively. For comparison, pure SnS₂ nanoparticles were also prepared according to the process described above with the exception of FGS as support.

Characterization

The dimension and morphology of the samples were observed by scanning electron microscopy (SEM) (Gemini LEO1530) and transmission electron microscopy (TEM) (JEOL JEM-2100). X-ray diffraction patterns were performed by a Bruker D8-Advance diffractometer equipped with graphite-monochromatized Cu Kα, radiation (λ = 0.15418 nm).

A two-electrode potentiostatic system with graphene-SnS₂ nanocomposites as the working electrode was used to carry out the electrochemical studies using lithium foil as the reference and counter electrodes. The working electrodes were prepared with graphene-SnS₂ nanocomposites as the active material and poly (vinylidene fluoride) as the binder, mixed by the weight ratio of 9:1, and dissolved in 1-methyl-2-pyrrolidinone to form a slurry. The mixture slurry was spread onto copper foil, dried in vacuum, and then pressed for use as the working electrode (the loading of pure active material was 2 mg/cm²). The coin cells were fabricated under an Ar-filled glove box, consisted of Li foil as the reference and counter electrodes and polyethylene film (Celgard 2300) as the separator. The electrolyte was 1 M LiPF₆ dissolved in an ethylene carbonate and diethyl carbonate (1:1 vol) mixture. The cells were charged and discharged at a constant current density of 200 mA/g between 0.01 and 1.2 V (vs. Li⁺/Li).

Results and discussion

Figure 1 shows the X-ray diffraction (XRD) patterns of FGS, GS1, GS2, GS3, and pure SnS₂ nanoparticles. The feature diffraction peak of FGS appears at 23° and the peak could also be observed in the figures of GS1, GS2, and GS3. The XRD pattern of pure SnS₂ matches well that of hexagonal SnS₂ (JCPDS card, no. 23-0677) and the diffraction peaks corresponding to (001), (100), (101), (110), and (111) planes of hexagonal SnS₂ could also be observed in GS1, GS2, and GS3.

Fig. 1

XRD patterns of FGS, GS1 (FGS to SnS₂ = 1:1), GS2 (FGS to SnS₂ = 1:2), GS3 (FGS to SnS₂ = 1:4), and pure SnS₂ nanoparticles

Figure 2a, e shows the morphology characterization of FGS. Both images indicate that the sample has wrinkled paper-like structure and the nanopores of FGS could be observed in Fig. 2a. In the SEM images of GS1 (Fig. 2b), GS2 (Fig. 2c), and GS3 (Fig. 2d), the wrinkles of FGS could also be observed. Besides, SnS₂ particles were observed to be intercalated into the nanopores of FGS. SnS₂ nanoparticles were well spread out on graphene sheets in all the three samples. The homogeneous distribution of SnS₂ particles on graphene sheets could contribute to buffering the volume expansion of SnS₂ particles during the charge and discharge processes. Subpanels f and g in Fig. 2 are TEM images of GS1 and GS3, respectively. In Fig. 2f, round and plate-shaped SnS₂ nanoparticles with an average size of about 30 nm were observed to be decorated on graphene. Besides, the figure demonstrates relatively low loading of SnS₂ in GS1. On the contrary, the TEM image of GS3 (Fig. 2g) shows that the composite consisted of graphene sheets heavily decorated with SnS₂ nanoparticles. Due to high loading of SnS₂ in GS3, there were hardly large areas of graphene sheets without SnS₂ decoration. By comparing Fig. 2e with Fig. 2g, it is obvious that the restacking of graphene sheets was effectively prevented with SnS₂ nanoparticles intercalated between graphene sheets. The high active surface area of graphene was therefore kept, which is favorable for increasing the lithium storage capacity of graphene in the composites [13]. The HRTEM image of GS1 (Fig. 2h) shows the lattice spacing of about 0.59 nm, which corresponds to (001) plane of hexagonal SnS₂.

Fig. 2

SEM images of a FGS, b GS1, c GS2, and d GS3; TEM images of e FGS, f GS1, and g GS3; h HRTEM image of GS1

Figure 3 shows the first 2 cycle’s charge–discharge curves of FGS, GS1, GS2, GS3, and pure SnS₂, which were measured between 0.01 and 1.2 V vs. Li⁺/Li at a current density of 200 mA/g. The red and green curves represent first and second charge–discharge process, respectively. A two-step electrochemical reaction mechanism of SnS₂ with lithium has been proposed as reported [14, 15]:

$$ {\text{Sn}}{{\text{S}}_{{2}}} + 4{\text{L}}{{\text{i}}^{{ + }}} + 4{e^{ - }} \to {\text{Sn}} + 2{\text{L}}{{\text{i}}_{{2}}}{\text{S}} $$

(1)

$$ {\text{Sn}} + x{\text{L}}{{\text{i}}^{ + }} + x{e^{ - }} \leftrightarrow {\text{L}}{{\text{i}}_x}{\text{Sn}}\left( {0 \leqslant x \leqslant 4.4} \right). $$

(2)

Fig. 3

Charge–discharge curves of a FGS, b GS1, c GS2, d GS3, and e pure SnS₂ at a current density of 200 mA/g

In the first discharge process of GS1, GS2, GS3, and pure SnS₂, the irreversible plateau emerged at about 1.2 V. It was assigned to the decomposition of the SnS₂ nanoparticles into Sn and Li₂S as shown by reaction 1. The discharge plateaus in the potential ranges of 0.0–0.5 V represent the formation of Li/Sn alloys and charge plateaus in the potential ranges of 0.5–0.7 V represent the dealloy process, which corresponds to reaction 2. The additional peak at 1.8 V in the first discharge process of samples GS1, GS2, and GS3 is attributed to the lithium intercalation into the SnS₂ layers without phase decomposition. It is possible that the homogeneous distribution of SnS₂ nanoparticles on graphene layers gives rise to high surface area of the particles, which promotes side reactions with the electrolytes and thus leads to lithium intercalation into the SnS₂ layers without phase decomposition. However, the peak was not obvious in the curve of pure SnS₂. It is supposed to be due to the aggregation of pure SnS₂ particles, which reduces their surface areas. In terms of the first discharge curve for FGS, an irreversible plateau was observed at about 0.7 V and it was also observed in the first discharge curves of GS1, GS2, and GS3. It may be related to the formation of a passivation film or solid electrolyte interphase [7, 9].

The initial discharge capacities for GS1, GS2, GS3, and pure SnS₂ were as high as 1,240, 1,420, 1,310, and 963 mAh/g, respectively. It may be due to the high surface-to-volume ratio of the nanostructure and the abundant surface defects, which may accelerate lithium intercalation and side reactions with the electrolytes. The initial efficiencies of these electrodes were calculated to be 21.1%, 23.8%, 29.6%, and 20.1%, respectively. It can be seen that the initial efficiencies increase with the increasing of SnS₂ amount in the composites and the initial efficiency of pure SnS₂ was the minimum among the four samples.

Figure 4 shows the electrochemical cycle properties of FGS, GS1, GS2, GS3, and pure SnS₂ over 50 cycles at a current density of 200 mA/g. It demonstrates that FGS had a good reversible capacity retention (83.1% after 50 cycles); however, the reversible capacity was only about 70 mAh/g. After 50 cycles, the reversible capacity of pure SnS₂ decreased to 23 mAh/g from the initial reversible capacity of 194 mAh/g with the retention of only 11.9%. This could be explained by its large volume changes in the processes of intercalation and deintercalation of lithium ion, which resulted in the pulverization and capacity fading. In contrast to the poor cycle performance of pure SnS₂, the reversible capacities of the graphene-SnS₂ nanocomposites were stable over 50 cycles. The initial reversible capacities for GS1, GS2, and GS3 were 261, 338, and 388 mAh/g, respectively. After 50 cycles, the reversible capacities decreased to 205, 286, and 351 mAh/g, corresponding to 78.5%, 84.6%, and 90.5% retention of the initial reversible capacities for samples GS1, GS2, and GS3, respectively. It can be seen that with FGS as a conductive matrix, the capacities of the samples increase with the increasing of SnS₂ amount in the composites. The improvement in the cycle performance of graphene-SnS₂ nanocomposites compared with pure SnS₂ could be attributed to the buffering and binding effect of FGS on the volume changes of SnS₂ in the charge and discharge processes.

Fig. 4

Variation of specific capacity with cycle number at a current density of 200 mA/g

To further study the rate capability of the samples, GS1, GS2, GS3, and FGS were tested with variable rates (from 100 to 1,000 mA/g) as shown in Fig. 5. At 1,000 mA/g, GS1, GS2, GS3, and FGS retained about 55%, 65%, 75%, and 60% of the initial capacity at 100 mA/g, respectively. It demonstrates that GS3 had the highest charge capacity among the four samples at variable rates and the loss of charge capacity of GS3 was the slowest. It is interesting to note that the rate capability of GS2 and GS3 was higher than that of FGS, besides, the capacity retention of GS2 and GS3 was also higher than that of FGS (Fig. 4). It could be attributed to the synergetic effects between graphene sheets and SnS₂ [13], which can be explained as follows: as two-dimensional matrix, graphene sheets guarantee the homogeneous distribution of SnS₂ nanoparticles and buffer the volume changes of SnS₂. Besides, the good conductivity of graphene sheets makes them good conductive channels. Meanwhile, the SnS₂ nanoparticles between graphene sheets prevent the restacking of graphene sheets effectively, which guarantees the efficient utilization of the good properties of graphene sheets. Graphene sheets and SnS₂ nanoparticles act upon each other and thus results in the enhanced elecrochemical properties of the composites.

Fig. 5

Rate capability of GS1, GS2, GS3, and FGS

Conclusion

In summary, graphene-SnS₂ nanocomposites with different weight ratios between graphene and SnS₂ were synthesized via a solvothermal method. The SEM and TEM characterizations revealed homogeneous distribution of SnS₂ particles on graphene sheets. Electrochemical tests of the composites as lithium-ion battery anode materials demonstrated that the capacity and cycle performance of the samples increased with the increasing of SnS₂ amount in the composites. The improvement of electrochemical properties of the composites compared with pure SnS₂ was attributed to the synergetic effects between graphene sheets and SnS₂.