Article pubs.acs.org/JPCC
A Few-Layer SnS2/Reduced Graphene Oxide Sandwich Hybrid for Efficient Sodium Storage Fengzhang Tu,†,‡ Xin Xu,† Pengzi Wang,† Ling Si,† Xiaosi Zhou,*,† and Jianchun Bao*,† †
Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡ College of Chemistry and Materials Science, Longyan University, Longyan 364012, P. R. China S Supporting Information *
ABSTRACT: Rechargeable sodium-ion batteries have lately received considerable attention as an alternative to lithium-ion batteries because sodium resources are essentially inexhaustible and ubiquitous around the world. Despite recent reports on cathode materials for sodium-ion batteries have shown electrochemical activities close to their lithium-ion counterparts, the major scientific challenge for sodium-ion batteries is to exploit efficient anode materials. Herein, we demonstrate that a hybrid material composed of few-layer SnS2 nanosheets sandwiched between reduced graphene oxide (RGO) nanosheets exhibits a high specific capacity of 843 mAh g−1 (calculated based on the mass of SnS2 only) at a current density of 0.1 A g−1 and a 98% capacity retention after 100 cycles when evaluated between 0.01 and 2.5 V. Employing ex situ high-resolution transmission electron microscopy and selected area electron diffraction techniques, we illustrate the high specific capacity of our anode through a 3-fold mechanism of intercalation of sodium ions along the ab-plane of SnS2 nanosheets and the subsequent formation of Na2S2 and Na15Sn4 through conversion and alloy reactions. The existence of RGO nanosheets in the hybrid material functions as a flexible backbone and high-speed electronic pathways, guaranteeing that an appropriate resilient space buffers the anisotropic dilation of SnS2 nanosheets along the ab-plane and c-axis for stable cycling performance. capacities of lower than 420 mAh g−1.17−19 Silicon, although the theoretical presence of Na−Si alloys, is electrochemically poor as well.20−22 Other group IV elements including germanium (NaGe, 369 mAh g−1),23 tin (Na15Sn4, 847 mAh g−1),24,25 and lead (Na15Pb4, 485 mAh g−1),26 and group V elements involving antimony (Na3Sb, 660 mAh g−1)27−32 and phosphorus (Na3P, 2596 mAh g−1),33−36 have been proposed as attractive anode candidates; however, they suffer from either low specific capacity or short cycle life (Table S1). SnS2 reacts electrochemically with sodium to produce Na2S2 and Na15Sn4 at a favorable potential as an anode material and with a large theoretical specific capacity of 843 mAh g−1, which greatly surpasses that of many other sodium-ion battery anode currently reported.37−40 SnS2 has a CdI2-type of layered structure (a = b = 3.65 Å, c = 5.90 Å, space group P3m1) composed of a layer of tin atoms sandwiched between two layers of hexagonally close-packed sulfur atoms.41 Considering appearance and structure, SnS2 is similar to graphite: it is flaky and consists of wavy sheets of covalently bonded tin and sulfur
1. INTRODUCTION Rechargeable sodium-ion batteries have recently attracted a great deal of attention as a promising alternative to lithium-ion batteries due to the abundant resources and low cost of sodium.1−3 Recent reports have demonstrated that the electrochemical performances of sodium-ion battery cathodes are comparable to their lithium-ion battery counterparts.4−13 Therefore, the significant challenge for sodium-ion batteries exists in developing a novel anode material with a high specific capacity and a suitable redox potential. Although various lithium-ion battery anode materials have been adapted to sodium-ion batteries, most of the attempts are unsatisfactory. Metal sodium is not a proper material owing to dendrite generation and its relatively low melting point (98 °C),14 and graphite is proved to be electrochemically sluggish because of the huge size of the sodium ions (2.04 Å) in comparison with the narrow channel size of the graphene sheets (1.86 Å).15 It has been reported that the average spacing between graphene layers can be widened to 3.7 Å and the sodium ions intercalate reversibly, with high capacity retention.16 Nevertheless, only a limited specific capacity of 251 mAh g−1, inadequate for practical applications, was obtained. Other carbon anode materials, such as hard carbon and soft carbon, deliver specific © 2017 American Chemical Society
Received: December 17, 2016 Revised: January 24, 2017 Published: February 2, 2017 3261
DOI: 10.1021/acs.jpcc.6b12692 J. Phys. Chem. C 2017, 121, 3261−3269
Article
The Journal of Physical Chemistry C
for 2 h under argon flow with a heating rate of 5 °C min−1 to obtain the product of FL-SnS2/RGO hybrid. 2.3. Structural Characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken by a JEOL JEM-2100F transmission electron microscope operated at 200 kV. Atomic force microscopy (AFM) was performed on a Digital Instruments NanoScope IIIa atomic force microscope by tapping mode. For AFM analysis, the dispersion of SnS2 nanosheets was added dropwise onto a fresh mica surface which was blow-dried with nitrogen. X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max 2500/PC diffractometer with Cu Kα irradiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) images were collected by a JEOL JSM-7600F scanning electron microscope operated at 10 kV. Scanning transmission electron microscopy (STEM) and elemental mapping analysis were carried out on the JEOL JEM-2100F transmission electron microscope equipped with a Thermo Fisher Scientific energy-dispersive X-ray spectrometer. Raman spectra were obtained on a Labram HR800 with a laser wavelength of 514 nm. X-ray photoelectron spectroscopy (XPS) was determined on an ESCALab250Xi electron spectrometer from VG Scientific using 300 W Al Kα radiation. The Brunner−Emmet−Teller (BET) specific surface area was measured with an ASAP 2050. Thermogravimetric analysis (TGA) was conducted on a NETZSCH STA 449 F3 in air atmosphere with a temperature ramp of 10 °C min−1 from room temperature to 800 °C. 2.4. Ex Situ HRTEM and SAED Observations. The ex situ HRTEM and SAED characterizations were performed on the JEOL JEM-2100F transmission electron microscope operated at 200 kV. The cells were disassembled after discharging down to different voltages. The resultant cycled electrodes were washed with dimethyl carbonate (DMC) several times and then dispersed in DMC under the protection of argon atmosphere by sonication. The cycled electrode material samples were prepared by pipetting a few microliters of the dispersion onto carbon grids purchased from Beijing Xixing Braim Technology Co., Ltd., in an argon-filled glovebox. The naturally dried carbon grids were put in sample holder for ex situ HRTEM and SAED experiments. Note that the cycled electrode materials were exposed to air for about 3 s during insertion of the holder into the TEM. 2.5. Electrochemical Characterization. Electrochemical performance was investigated by galvanostatic cycling of CR2032 coin cells with the FL-SnS2/RGO hybrid as the working electrode, glass fiber (GF/D) from Whatman as the separator, and sodium metal as the counter electrode. The working electrodes were prepared using a typical slurry method with FL-SnS2/RGO powders and carboxymethylcellulose sodium (CMC) binder with a mass ratio of 9:1 in deionized water solvent. The mass loading of active material (SnS2 nanosheets) is ∼0.8 mg cm−2, corresponding to a total mass loading of ∼1.27 mg cm−2. The thickness of the electrode except the current collector was 9.3 ± 0.5 μm. The electrolyte solution was 1 M NaClO4 in ethylene carbonate (EC)−diethyl carbonate (DEC)−fluoroethylene carbonate (FEC) (1:1:0.1 in volume). The coin cells were assembled in an argon-filled glovebox (H2O, O2 < 0.1 ppm, MBraun, Germany). Electrochemical data were recorded on a Land CT2001A multichannel battery test system at various current densities in the voltage range of 0.01−2.5 V vs Na/Na+ at room temperature. The specific capacity was computed based on the mass of SnS2. The mass loading of active material was further increased to ∼2.0
atoms.42 However, in comparison with graphite, SnS2 possesses a greater interlayer channel size (2.26 versus 1.86 Å), implying that sodium (2.04 Å) ions can be easily stored between the SnS2 layers. While SnS2 has been already reported in the literature as a high-capacity anode material for lithium-ion batteries,43−46 its unsatisfactory specific capacity or cycling stability for sodium-ion batteries requires to be enhanced.47−50 It is thereby essential to study the sodiation mechanism in order to improve the performance of SnS2 for utilization in sodium-ion batteries. In this work, we study the sodiation mechanism of SnS2 using ex situ high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) characterizations. A three-step sodiation mechanism of intercalation and conversion as well as alloying is revealed. A large anisotropic volumetric variation along the c-axis is demonstrated during the sodiation process, which results in a rapid capacity decline upon cycling. To realize both high capacity and excellent cycling operation, we have especially designed a few-layer SnS2/reduced graphene oxide (FL-SnS2/ RGO) hybrid with plenty of SnS2 nanosheets sandwiched between RGO nanosheets. The features of our structure design are triple: (1) The RGO nanosheets serve as a flexible cushion layer to buffer the anisotropic volumetric expansion during sodiation. (2) The few-layer SnS2 nanosheets provide a shortened diffusion distance for sodium ions. (3) The RGO layers work as high-speed electronic pathways; thus, the resulting hybrid material is endowed with a high electrochemical activity.
2. EXPERIMENTAL SECTION 2.1. Synthesis of SnS2 Nanosheets and Graphene Oxide (GO). Both SnS2 nanosheets and GO were synthesized by a liquid-phase exfoliation approach according to the procedure reported previously.51 SnS2 particles were prepared from SnCl4·5H2O and thioacetamide by a hydrothermal method.41 The as-prepared SnS2 particles were dispersed in deionized water (cylindrical tube, 20 mL of H2O) to create a 5.0 mg mL−1 dispersion and sonicated in a sonic bath (KQ3200DE Ultrasonic) for 30 min. Graphite oxide was prepared from natural graphite by a modified Hummers’ method52 and was also dispersed in deionized water (30 mL of H2O) to form a 1.0 mg mL−1 dispersion and sonicated in a sonic bath for 10 min. The obtained dispersion of SnS2 nanosheets was centrifuged using a Techcomp CT-14RDII centrifuge for 5 min at 13 500 rpm to remove precipitate and collect the supernatant, while the resulting dispersion of GO was centrifuged at 3000 rpm for 30 min. After centrifuging, decantation was performed by pipetting off the top threequarters of the dispersion. These processes lead to SnS2 nanosheets and GO yields (mass of SnS2 nanosheets or GO/ starting material mass) of 23 and 95 wt %, respectively. The precipitates of SnS2 could be reused to fabricate SnS2 nanosheets by repeating the aforementioned procedures. 2.2. Synthesis of FL-SnS2/RGO Hybrid. Dispersions of SnS2 nanosheets and GO were mixed uniformly with different volume ratios by sonicating to achieve different SnS2/GO molar ratios (Table S3). Because of the gradual oxidation of SnS2 in air, the sandwiched FL-SnS2/GO was synthesized by rapidly freezing the dispersion in liquid nitrogen and subsequent freeze-drying using a FD-1A-50 freeze-drier. The resultant sample was placed into a ceramic boat, transferred into a temperature-programmed furnace, and then heated to 300 °C 3262
DOI: 10.1021/acs.jpcc.6b12692 J. Phys. Chem. C 2017, 121, 3261−3269
Article
The Journal of Physical Chemistry C
Figure 1. Sodiation mechanisms of the sandwiched FL-SnS2/RGO structure. Schematic illustration of the FL-SnS2/RGO sandwich hybrid before sodiation, with the first step of sodium-ion insertion, and the second step of conversion and alloy reactions to generate Na2S2 and Na15Sn4, respectively.
Figure 2. Ex situ HRTEM images and SAED patterns of SnS2 particles taken before sodiation and after discharging to different voltages. (a, e) Before sodiation. (b, f) After discharging to 1.72 V. Sodium ions diffusion along the ab-plane channels, resulting in a volume expansion along the c-axis direction. (c, g) After discharging to 1.69 V. (d, h) After discharging to 0.01 V.
mg cm−2 for further investigation of its electrochemical performance. The thickness of the electrode except the current collector was 21.7 ± 0.6 μm. The rate capability and cycling performance are shown in Figure S13.
of 1.69−2.5 V (Figure S2a,b). However, the specific capacity is restricted to a low value of 10 mAh g−1, i.e., Na0.07SnS2. Further sodiation below 1.69 V leads to the generation of NaxS (conversion reaction) and NaxSn species (alloy reaction), with the disappearance of the representative SnS2 lattice fringes (Figure 2a−c) and diffusive rings (Figure 2e−g). Lattice fringes of the intermediate NaxS and NaxSn phases are hard to recognize because of poor crystallinity, small domain size, and the particle distribution within the conductive additive matrix. Nonetheless, when the voltage decrease to 0.01 V and the conversion and alloy reactions approach to completion, a set of parallel fringes with typical d-spacing of 2.2 and 4.6 Å can be assigned to the (300) plane of Na2S2 (PDF 01-081-1771) and the (220) plane of Na15Sn4 (JCPDS card No. 31-1327), respectively (Figure 2d). The copresence of Na2S2 and Na15Sn4 crystal phases is also confirmed by SAED (Figure 2h). Therefore, the conversion and alloying mechanisms principally explain the high specific capacity of SnS2 (Figure S2e,f). Nevertheless, this mechanism also causes a huge volume variation of 344% (Table S2), which brings about mechanical fracture and loss of electrical contact (Figure S3), resulting in drastic capacity decay during cycling (Figure S2d,f). These challenges resemble to those with other high-capacity materials such as phosphorus33−36 and sulfur.54,55 It is thus essential to comprehend the nature of the stress evolution in SnS2 in order to effectively solve these problems by
3. RESULTS AND DISCUSSION The storage of sodium ions in SnS2 (Figure S1) undergoes three different reaction mechanisms including insertion, conversion, and alloying, as demonstrated in Figure 1. Sodium ions first intercalate between the SnS2 layers (insertion), along the ab-plane-oriented channels. Clearly, the passages along the ab-planes (2.26 Å) are broad enough to permit the diffusion of sodium ions (2.04 Å). An ex situ HRTEM image of SnS2 before sodiation (Figure 2a) displays distinctive lattice fringes with an average interplanar distance of 6.0 Å, corresponding to the dspacing of the (001) crystal plane. These characteristic lattice fringes become larger with the sodium-ion intercalation into SnS2 (Figure 2b), meaning that the distance between the SnS2 layers along the c-axis has increased and indicating the insertion of sodium ions. The lattice fringes are obvious until the potential diminishes to 1.69 V (Figure 2c), suggesting that the layered structure is maintained upon sodiation. This mechanism resembles to the insertion process happening to graphite during lithiation, producing the LiC6 phase.53 The insertion does not remarkably change the host structure, thereby ensuring high reversibility during cycling in the voltage range 3263
DOI: 10.1021/acs.jpcc.6b12692 J. Phys. Chem. C 2017, 121, 3261−3269
Article
The Journal of Physical Chemistry C
Figure 3. Evidence of few-layer SnS2 nanosheets. (a) TEM image of SnS2 nanosheets. (b) HRTEM image of a SnS2 nanoplate. (c) HRTEM image of selected area of (b) with marked interlayer distance. (d) HRTEM image of the edge of a SnS2 nanosheet. (e) AFM image of SnS2 nanosheets. (f) Measured thickness of the SnS2 nanosheets in (e). (g) XRD patterns of SnS2 particles and SnS2 nanosheets. (h) Magnification of XRD patterns of SnS2 particles and SnS2 nanosheets in the low-angle region (dotted rectangle in (g)).
the SnS2 nanosheets offers a flexible cushion to buffer anisotropic expansion. Few-layer SnS2 nanosheets were peeled off using a technique according to a previously reported approach that is solutionbased and adaptable,51 making it easily scalable, and with a yield close to 23 wt % (see Experimental Section). The as-formed few-layer SnS2 nanosheets were imaged through TEM (Figure 3a) and atomic force microscopy (AFM) techniques (Figure 3e,f). Their thicknesses vary from 4.22 to 6.75 nm, corresponding to five to eight SnS2 layers (scarcely does the thickness surpasses eight layers). The side length of each SnS2 nanosheet ranges from 10 to 30 nm. The HRTEM image (Figure 3d) displays a nanosheet consisting of eight SnS2 layers with a [001] interlayer spacing of 6.0 Å, which is slightly larger than the 5.9 Å of bulk SnS2, as corroborated by XRD patterns (Figure 3g,h). Accordingly, the ab-plane channel sizes increase to 2.36 Å, expanding the channels for sodium ions (2.04 Å) diffusion. The FL-SnS2/RGO sandwich structure was obtained simply by mixing aqueous suspensions of SnS2 nanosheets and GO (Figure S4), followed by self-assembly after freeze-drying and subsequent thermal reduction (see Experimental Section). The SEM image in Figure 4a,b shows the high quality of homogeneous FL-SnS2/RGO sandwich nanosheets. The electron beam could pass through the carbon shells, verifying the sandwiched structure of FL-SnS2/RGO with a particle size of several micrometers (outside length) (Figure 4b,c). The TEM image further discloses that the SnS2 yolks ∼20 nm in size are crystalline particles and uniformly encapsulated by the RGO shells with void space in between (Figure 4c). Additionally, the SAED pattern in the inset of Figure 4c presents a series of diffusive rings with interlayer distance of 3.2, 2.8, and 1.8 Å, respectively, corresponding to the (100), (101), and (110) planes of single crystalline SnS2. The HRTEM image of the edge demonstrated in Figure 4d shows SnS2 nanosheets lying between larger RGO nansheets, with alternating SnS2 and RGO layers. The STEM and EDX mapping images (Figure
designing advanced nanostructured materials. Figure 3a shows a TEM image of SnS2 nanosheets obtained by liquid-phase exfoliation of SnS2 particles. Interestingly, we notice many hexagonal objects among the common SnS2 nanosheets (Figure 3a−c). All the hexagons are also sharply outlined and stacked along the c-axis direction, as demonstrated in the HRTEM image (Figure 3d). Actually, cutting along the a-axis or b-axis needs a large number of Sn−S bonds to be fractured, both of which have the same bond length (2.38 Å) and therefore equal. It would be impossible to achieve such a sharply outlined hexagon if only cutting only along the a-axis or b-axis direction. The galvanostatic sodiation of the SnS2 particles (Figure 2a) took place as they were assembled into the sodium-ion battery electrode. Figure 2b,c presents the SnS2 particles after discharging to 1.72 and 1.69 V, respectively. A 25% dilation along the c-axis was observed. The almost same expansions along the a-axis and b-axis directions are the result of identical sodium ion diffusion and the unanimous Sn−S bonding along the ab-plane. The complete sodiation in SnS2 yielding Na2S2 and Na15Sn4 possesses a theoretical volume expansion as high as 344%, and the c-axis expansion has been estimated to be ∼25%. On the basis of these results, we deduce that obvious expansions (∼66%) along the a- and b-axis also occur. Therefore, decreasing the thickness and size of the SnS2 plates would help diminish the strain accumulated along the c-axis and ab-plane directions. In this regard, an electrode composed of monolayer SnS2 nanosheet would be perfect.56,57 Broadly, fewlayer (
