One-Step Synthesis of SnS2 Nanoflower/Graphene Nanocomposites with Enhanced Lithium Ion Storage Performance

TIAN Ai-Hua WEI Wei QU Peng,* XIA Qiu-Ping SHEN Qi



One-Step Synthesis of SnS2Nanoflower/Graphene Nanocomposites with Enhanced Lithium Ion Storage Performance

TIAN Ai-Hua1,2WEI Wei1,2QU Peng1,2,*XIA Qiu-Ping2SHEN Qi1,*

(1;2)

SnS2is considered as an attractive anode material to substitute commercial graphite anodes of lithium-ion batteries due to its high specific capacity of 645 mAh·g−1as well as low cost. Nevertheless, it suffers poor large volume expansion during the lithiation/delithiation processes, leading to the loss of electrical contact and rapid capacity fading. Herein, by using a facile one-step solvothermal method, SnS2nanoflower/graphene nanocomposites (SnS2NF/GNs) were prepared, where flower-like SnS2hierarchical nanostructures consisting of ultrathin nanoplates, are tightly enwrapped in graphene nanosheets. As anode materials for lithium-ion batteries, the SnS2NF/GNs electrode exhibit superior electrochemical performance, with a reversible capacity of 523 mAh·g−1after 200 charge-discharge cycles. The enhanced Li storage performance was attributed to the synergistic effect of SnS2and graphene. The SnS2NF can effectively accommodate the volume change and shorten Li+diffusion distance, while graphene nanosheets can further alleviate the volume expansion of SnS2and improve the electronic conductivity.

Lithium-ion battery; Anode; SnS2nanoflower; Graphene

1 Introduction

Rechargeable lithium-ion batteries (LIBs), as advanced energy-storage devices, have received tremendous attention due to their outstanding performance of long cycle lifetime, high specific energy density as well as environmental benignity and their wide applications ranging from consumer electronics to various electric vehicles1−7. Electrode materials, which are the most important component in LIBs, largely influence the performance of batteries. The commercial graphite anode has the advantages including low cost and high electrical conductivity8−11, while suffering low theoretical specific capacity (only 372 mAh·g−1at full lithiation)12,13, inferior reaction kinetics and potential safety problems14. Therefore, the development for alternative materials with high reversible capacity, excellent rate performance and safety aroused great interest.

In recent years, Sn-based materials (Sn, SnO2, SnS2) have been considered as promising anode candidates for the next-generation LIBs because of their high theoretical specific capacities, low cost and suitable working potential12,15−18. Especially, CdI2-type tin sulfide (SnS2), attracts more interest due to its unique layered crystal configuration15,16,19,20. Unfortunately, the practical application of SnS2in LIBs is restricted by the poor cycle stability, which is attributed to its huge volume expansion (> 300%) that accompanies the charge-discharge processes19,21,22, causing the increasing mechanical stress and pulverization and exfoliation of active material from the current collector and thus resulting in serious capacity fading and poor cyclability. To address this issue, extensive studies have been focused on the preparation of nanoscale SnS220,23−26, such as zero-dimension (0D) nanoparti- cles, one-dimensional (1D) nanostructures, and two-dimension- al (2D) nanomaterials, because of the advantages of decreasing huge volume changes, high electrolyte contact area and shortening diffusion pathways for electrons and ions27−30. Although having apparently improved the electrochemical performance of SnS2anode in LIBs, many questions still remain for low-dimensional nanostructured SnS2. For example, due to the high surface energy, 0D nanoparticles are likely to suffer from self-aggregation after long cycles, which lead to structural instability and poor capacity retention30−32. The fabrication procedures of 1D nanostructures generally involved template-based approach that are related to sophisticated processes and high cost33,34. While, 2D nanomaterials tend to stacking due to the weak van der Waals forces between layers, thus reducing the specific surface area24. One effective approach is to constructing two-dimensional (3D) hierarchical nanostructures, which not only retain the advantages of low-dimensional nanostructures, but also avoid the self-aggregation of active nanomaterials and provide high porosity that can effectively alleviate the stress induced during cycling process, thereby ensuring structural stability of the whole architecture29,35−39.

To further optimize the electrochemistry, assembling SnS2nanosheets into 3D hierarchical porous nanostructures may be the best strategy. In such structures, the stacking of SnS2nanosheet could effectively be avoided. In addition, building porous structure could provide more buffer space and keep larger specific surface area. However, since SnS2has low electrical conductivity, 3D SnS2architectures are not sufficient to achieve high performance electrode materials. The hybridization between 3D hierarchical nanostructured SnS2and carbonaceous material perhaps is an ideal choice. Graphene, a two-dimensional (2D) sheet ofsp-hybridized carbonaceous materials, is considered as to be an ideal matrix because of its many good properties, such as large surface areas (2620 m2·g−1), superior mechanical strength and flexibility, excellent chemical stability and high intrinsic carrier mobility40−43. What's more, since similar to SnS2on the structure and morphology, graphene is more likely intercalate to sandwich structured SnS2than other carbonaceous materials, greatly reducing the stacking of SnS2, which could be favorable to faster Li ion and electron transport and provide high electrolyte contact area.

Based on this concept, a novel nanocomposite of flower-like SnS2enwrapped in conductive graphene sheets was successfully prepareda simple one-step solvothermal method. In such a hybrid structure, flower-like SnS2was assembled by ultrathin nanoplates (about 2.4 nm), forming a porous 3D hierarchical structure. When used as the anode for LIBs, the nanocomposites exhibited enhanced lithium ions storage performance.

2 Experimental section

2.1 Synthesis of SnS2 NF/GNs

Graphene oxide (GO) was fabricated from natural graphite powder by a modified Hummers method44. In a typical synthesis, the as-prepared GO (0.2000 g) was firstly ultrasonic dispersed in ethylene glycol (80 mL) (> 99%, GC) to form black homogeneous dispersion. Subsequently, 0.5211 g SnCl4(AR) and 0.1522 g thiourea (Tu) (AR) were added into the above solution. After 20 min, the mixed solution was transferred into a stainless Teflon-lined autoclave (100 mL) and maintained at 180 °C for 20 h. After cooling to room temperature naturally, the black SnS2NF/GNs powder was obtained by centrifugation, washed with deionized water for six times and freeze-dried at −80 °C for 16 h. SnS2nanoflower (SnS2NF) powders were synthesized by the same process without GO.

2.2 Characterization

The microstructure and morphology of the as-prepared materials were studied using a scanning electron microscope (SEM) (Hitachi S-7500, Japan, 5 kV), transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED) as well as high-resolution transmission electron microscopy (HRTEM) (JEM-2100F, JEOL Ltd., Japan, 20 kV). The crystal structure and phases of the samples were carried out with powder X-ray diffraction (XRD) measurements on a Rigaku D/max-2200 X-ray diffractometer (Japan) with CuKradiation (radiation wavelength= 0.154 nm) over the scattering angles range of 10°−80° at a scan rate of 6 (°)·min−1. Thermogravimetric analysis (TGA) was performed on a Netzsch instrument TG/STA 449F3 (Germany) in an air atmosphere at a heating rate of 10 °C·min−1.

2.3 Electrochemical tests

The fabrication process of LIBs was as follows. 80% (, mass fraction) SnS2NF/GNs (or SnS2NF), 10% () of conducting additive (carbon black), and 10% () polyvinyl- idene fluoride (PVDF) (AR) were added to 1-methyl-2-pyrrol- idene (NMP) (AR) solution to form a homogeneous slurry, which was then uniformly coated onto a copper foil current collector and dried in vacuum at 60 °C for 12 h to form working electrode. The mass loading of the active material for each current collector was about 2 mg·cm−2. The CR2016-type coin cells were assembled in an argon-filled glove box with lithium metal and Celgard 2400 membranes as the counter electrodes and the separator, respectively. The electrolyte was made of 1 mol·L−1LiPF6in ethylene carbonate (EC) and dimethyl carbonate (DMC) with the volume ratio 1:1. Galvanostatic charge-discharge tests were performed on a Land BT2013A battery testing system (Wuhan, China) at a voltage interval of 0−1.5 V (Li+/Li). Cyclic voltammetry (CV) curves (scan rate of 0.1 mV·s−1) and electrochemical impedance spectroscopy (EIS) (amplitude: 5 mV, frequency: 0.01−100 kHz) were recorded on a CHI-660D electrochemical workstation (Chenhua Co., Ltd., Shanghai, China). All electrochemical tests were conducted at room temperature.

3 Results and discussion

The preparation of SnS2NF/GNs is schematically illustrated in Scheme 1. When SnCl4was added to a GO suspension, Sn4+ions were firstly adhered onto the surface of GO sheets through electrostatic interaction, and then coordinated with the added thiourea (Tu), forming Sn-Tu complex, in which Tu was not only used as complexing agent, but also as sulfur sources. During the solvothermal process, SnS2nanoplates were formed and assembled into SnS2nanoflowers by the pyrolysis of Sn-Tu complex, simultaneously, GO was reduced to graphene. These SnS2nanoflowers were spontaneously and simultaneously encapsulated in the graphene networks. Thus, such a unique nanostructure, in which SnS2nanoflowers were encapsulated in the graphene networks, was formed.

Scheme 1 Schematic illustration of synthesis processes of SnS2NF/GNs.

The X-ray diffraction (XRD) pattern of as-fabricated SnS2NF/GNs sample is presented Fig.1(a). A diffraction peak located at 26.5° is observed, which is assigned to the characteristic (002) plane reflection of graphene, and the weak peak intensity may be related to the poor crystallinity and the lack of significant layer-to-layer stacking of graphene, which can be confirmed by TEM (Fig.1(g, h))42−45. All the other diffraction peaks are indexed to the (001), (100), (101), (102), (110) and (111) lattice planes of the rhombohedral phase of SnS2(JCPDS card No. 23-0677), and the sharp diffraction peaks imply that the achieved SnS2is well crystallized38. No other impurity peak can be observed in the patterns, indicating the pure phase of SnS2in the graphene matrix. These results indicate that the composite material comprises crystalline SnS2and disordered crystalline structures of graphene. The thermal gravimetric analysis (TGA) technique was performed to determine the graphene content in the SnS2NF/GNs nanocomposite (see Supporting Information Fig.S1†). The mass loss of 14.39% between 300 and 450 °C was due to the oxidation of SnS2to SnO221. At 450−700 °C, there is a mass loss of 20.83%, which was attributed to graphene combustion46.

The morphology and structures of two samples are illuminated by scanning electron microscopy (SEM). Fig.1(b) is a typical low-magnification SEM image of SnS2NF material, and it can be seen that it consists of a large quantity of 3D flower-like nanostructures, which exhibit a uniform morphology and have no tendency to agglomeration. Additionally, the high-magnification SEM view of the material demonstrates that the flower-like SnS2, constructed from curved ultrathin nanoflakes, has an average particle size of about 700 nm, as shown in the Fig.1c. It is noteworthy that there are some voids in the SnS2nanoflowers, which facilitate electrolyte diffusion and lithium ion transport44,47. When hybridized with graphene, the SnS2still maintains pristine flower-like morphology. Nevertheless, they are tightly wrapped by graphene, as revealed in Fig.1(d−f). TEM and HRTEM patterns are performed to further investigate the microstructureof the as-prepared SnS2NF/GNs. Fig.1(g, h) shows typical TEM images of the SnS2NF/GNs at different magnifications, which clearly indicate the SnS2nanoflowers are well encapsulated by transparent graphene nanosheets and have an average size about 700 nm, in accordance with SEM results. It is believed that the agglomeration of graphene between layers can be restrained on account of the anchoring of the SnS2particles, and thus, the high active surface area of graphene is well retained, which will be favorable for increasing the lithium storage capacity of graphene in the composites41,44,48. The ring-like SEAD pattern (inset in Fig.1(h)) is well indexed as a hexagonal SnS2phase, indicating the polycrystalline nature of the SnS2in the composites. The HRTEM image (Fig.1(i)) exhibits a characteristic lattice fringes with an interplanar spacing of 0.32 and 0.59 nm, which corresponds to (100) and (001) plane of hexagonal SnS2crystal, respectively19,49. This is consistent with the result obtained from the XRD. Furthermore, it is clear to see that nanoplates are composed of 4 sandwiched S-Sn-S, indicating that flower-like SnS2was assemble by ultrathin nanoplates46.

The electrochemical reaction mechanisms of lithiation/ delithiation for SnS2NF/GNs were investigated using cyclic voltammogram (CV), as shown in Fig.2(a). Upon the initial cathodic sweep, two peaks appearing at higher voltage (1.84 and 1.6 V) are generally attributed to Li+intercalation into the SnS2layers (i.e.,Li++ SnS2+e-= LiSnS2) without phase decomposition, which completely disappear in the following cycles, suggesting that it is an irreversible process4,21. A remarkable cathodic peak located at around 1.18 V could be related to the decomposition of SnS2into metallic Sn and Li2S (i.e., SnS2+ 4Li++ 4e-= Sn + 2Li2S) as well as the formation of a solid electrolyte interface (SEI), which may lead to large irreversibility of anode materials in the 1st cycle38,50. Another intense peak observed at 0.01−0.5 V corresponds to reversible lithiation of metallic Sn that forms Li4.4Sn alloys (i.e., Sn + 4.4Li++ 4.4e−= Li4.4Sn)19. In the corresponding anodic scan, the broad oxidation peak between 0.3 to 0.9 V can be attributed to the dealloying reaction of Li4.4Sn to Sn and Li16. In the following CV scans, the positions and intensities of the redox peaks remain unchanged substantially. For SnS2NF electrode (Fig.S2), similar reactions are produced. However, the peak current tremendously decreases, indicating that serious capacity fading occurred during the alloying and dealloying processes. The result implies the nanocomposite has good stability and reversibility in the processes of cycles.

Fig.1 (a) XRD pattern of the SnS2 NF/GNs; (b and c) SEM image of SnS2 NF under the different magnification; SEM (d, e and f),TEM images (g and h) and HRTEM micrograph (i) of the SnS2 NF/GNs. Inset of (h): SAED pattern.

Fig.2(b, c) exhibits the typical charge-discharge profiles of SnS2NF and SnS2NF/GNs tested in the voltage range of 0−1.5 V at a current density of 100 mA·g−1. As displayed in Fig.2(b), the SnS2NF sample delivers an initial discharge-charge capacity of 600.0 mAh·g−1and 59.4 mAh·g−1, with a coulombic efficiency (CE) of only 9.9%. The large irreversible capacity can be related to the large volume expansion of the SnS2, the decomposition of the electrolyte, partially reversible transformation of Li+layer on the electrode surface during the first cycle21,51−54. In contrast, as shown in Fig.2(c), the SnS2NF/GNs delivered an initial discharge capacity of 1830.3 mAh·g−1and charge capacity of 782.8 mAh·g−1, corresponding to a CE of 42.8%. It is worth noting that the CE of SnS2is obviously improved after adding graphene. Furthermore, from the second cycle, the CE of the electrode is improved significantly and reaches approximately 96% after 5 cycles. This can be attributed to the unique nanostructure of SnS2NF/GNs, in which elastic graphene sheets not only serve as a highly electrically conductive continuous medium, but also buffer the volume change of SnS2during the Li+insertion/ extraction processes.

Fig.2 (a) Cyclic voltammogram (CV) curves of the SnS2 NF/GNs at a scan rate of 0.1 mV s-1 of the first three cycles. Galvanostatic charge-discharge curves of (b) SnS2 NF and (c) SnS2 NF/GNs under the current density of 100 mA·g−1 in the voltage range of 0-1.5 V.(d) Cycling performance of two samples within a voltage range of 0-1.5 V (vs Li+/Li) at a current density of 100 mA·g−1.(e) Rate capability of the SnS2 NF/GNs electrode under different current densities. (f) Impedance Nyquist plots of the two electrodes after 200 cycles at the same current density in the fully charged state.

The cycling performances for the two samples at a constant current density of 100 mA·g−1are plotted in Fig.2(d). Here, all capacities were calculated on the basis of the total weight of composite material including SnS2and graphene. It can be found that the capacity of SnS2NF displays a rapid capacity decaying from an initial 600 mAh·g−1to merely 63 mAh·g−1after 200 cycles, which most likely originates from the limited volume accommodation of the SnS2NF during the process of cycles, thus resulting in the pulverization and electronic detachment with the current collector. For the pure graphene electrode, the capacity of which was 178 mAh·g−1after 80 cycles with an initial charge capacity of 192 mAh·g−1(Fig.S3†). In contrast, the SnS2NF/GNs electrode, under the same conditions, can maintain a specific capacity of 523 mAh·g−1, which is ~1.5 times higher than the theoretical capacity of graphite. The improved cycling performance can be attributed to the synergistic effect of flower-like SnS2and graphene matrixes. The ultrathin nanosheet in flower-like SnS2can increase the contact area with the electrolyte and shorten the diffusion length of Li ions. While, graphene matrixes can not only improve the electrical conductivity, but also effectively accommodate the large volumetric change of SnS2during electrochemical reaction processes.

The rate capabilities of SnS2NF/GNs were measured to further demonstrate the potential of SnS2NF/GNs as electrode for LIBs. As plotted in Fig.2(e), when the current density increases in stages from 136 to 680 and 1360 mA·g−1, the composite exhibits reversible capacities of 519, 362 and 211 mAh·g−1, respectively. Notably, the specific capacity is completely restored when the charge-discharge rate returns to the initial rate of 136 mA·g−1, implying that the SnS2NF/GNs electrode has good structural stability and high recovery ability. This result can be ascribed to the unique 3D conductive network structure, where each SnS2nanoflower is separated by nanosheets, manifesting that the aggregation and volumetric expansion during the cycling process of SnS2nanoflowers are effectively be avoided.

The enhanced electrical conductivity of the SnS2NF/GNs composites in comparison with that of SnS2NF can be confirmed by electrochemical impedance spectroscopy (EIS) measurements, as shown in Fig.2(f). The EIS plots for the two electrodes are composed of a depressed semicircle in the high frequency range and a sloping straight line in the low frequency region. The semicircle is related to the contact resistance, the SEI layer resistance (f) as well as charge transfer resistance (ct) between the active material and the electrolyte, and the slope line represents the Warburg impedance (w) of the Li ion diffusion in the anode. Apparently, the resistance of the SnS2NF/GNs is much smaller than that of SnS2NF, indicating that the former has a faster charge transfer at the electrode/ electrolyte interface and faster Li ion migration through the SEI film, which results in higher reversible capacity of SnS2/graphene composites in comparison with the pure SnS2nanoflowers. The improved electrochemical performance of SnS2NF/GNs is closely related to the following reasons. First, the SnS2has a large interlayer spacing, which is favourable to the insertion/extraction of lithium ions. Second, ultrathin nanoflakes shorten the diffusion length of lithium ion and improve the interfacial contact area between electrode/ electrolyte. Furthermore, the conductive graphene nanosheets not only facilitate electron transport in composite materials, enhancing the electrical conductivity of the electrode, but also accommodate the volume expansion as well as avoid the agglomeration of SnS2during the lithiation/delithiation process.

4 Conclusions

In conclusion, SnS2NF/GNs nanocomposites have been successfully fabricated through a facile one-step solvothermal approach, in which the flower-like SnS2comprised of ultrathin SnS2nanoflakes with an average particle size of about 700 nm was tightly encapsulated in graphene sheets. Benefiting from the synergistic effects of nanostructured SnS2and conductive graphene nanosheets, the nanocomposites exhibit superior electrochemical performance, for instance, good cycling stability (523 mAh·g−1for 200 cycle) and high coulombic efficiency (approximately 96% after 5 cycles). Thus, the SnS2NF/GNs composite is expected to be a high capacity anodic material for LIBs.

Supporting Information:available free of chargethe internet at http://www.whxb.pku.edu.cn.

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SnS2纳米花/石墨烯纳米复合物的一步法合成及其增强的锂离子存储性能

田爱华1,2魏 伟1,2瞿 鹏1,2,*夏修萍1,2申 琦1,*

(1郑州大学化学与分子工程学院，郑州 450001；2商丘师范学院化学化工学院，河南省生物分子识别与传感重点实验室，河南 商丘 476000)

SnS2由于具有较高的储锂容量(645 mAh·g−1)、价格低廉等优点而受到研究者的广泛关注。但纯SnS2在脱嵌锂过程中存在严重的体积膨胀效应，造成活性物质粉化和剥落，从而导致容量的迅速衰减。针对这一问题，本文采用简单的一步溶剂热法制备了SnS2纳米花/石墨烯(SnS2NF/GNs)纳米复合物。其中花状SnS2由超薄纳米片组装而成，石墨烯纳米片将SnS2包裹在其中。将该材料用作锂离子电池负极时，SnS2NF/GNs表现出优越的电化学性能，如：循环200圈后可逆容量仍可达523 mAh∙g−1复合物材料提高的储锂性能得益于SnS2和石墨烯的协同效应。纳米结构的SnS2可以有效的缓冲体积的膨胀，缩短锂离子的扩散距离。石墨烯纳米片不仅可以进一步缓冲SnS2体积的膨胀，而且可以提高纳米复合物的导电性。

锂离子电池；负极；SnS2纳米花；石墨烯

O643

10.3866/PKU.WHXB201704191

February 17, 2017;

April 12, 2017;

April 19, 2017.

Corresponding authors.QU Peng, Email: qupeng0212@163.com; Tel: +86-370-3112602; SHEN Qi, Email: shenqi@zzu.edu.cn.

The project was supported by the National Natural Science Foundation of China (21575131) and the Key Scientific Research Project of High Schools in Henan Province (16A430025, 17A480009).

国家自然科学基金(21575131)和河南省高等学校重点科研项目(16A430025, 17A480009)资助