Polypyrrole Ultrathin Nanosheets as High

Mar 17, 2016 - Sandwich-like SnS/Polypyrrole Ultrathin Nanosheets as High-Performance Anode Materials for Li-Ion Batteries ... School of Materials Sci...
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Sandwich-like SnS/Polypyrrole Ultrathin Nanosheets as HighPerformance Anode Materials for Li-Ion Batteries Jun Liu,†,§ Mingzhe Gu,‡ Liuzhang Ouyang,†,§ Hui Wang,†,§ Lichun Yang,†,§ and Min Zhu*,†,§ †

School of Materials Science and Engineering and Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, P. R. China ‡ Key Laboratory of Low Dimensional Materials & Application Technology, Ministry of Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, P. R. China § China-Australia Joint Laboratory for Energy & Environmental Materials, South China University of Technology, Guangzhou, 510641, P. R. China S Supporting Information *

ABSTRACT: Sandwich-like SnS/polypyrrole ultrathin nanosheets were synthesized via a pyrrole reduction and in situ polymerization route, in which room-temperature synthesized ZnSn(OH)6 microcubes were used as the tin source. As anode materials for Li-ion batteries, they exhibit an extremely high reversible capacity (about 1000 mA h g−1 at 0.1C), outstanding rate capability (with reversible capabilities of 878, 805, 747, 652, and 576 mA h g−1 at 0.2C, 0.5C, 1C, 2C, and 5C, respectively), stable cycling performance, and high capacity retention (a high capacity of 703 mA h g−1 at 1C after long 500 cycles). KEYWORDS: Li-ion batteries, anode materials, conductive polymer, 2D sandwich structure, tin sulfide

1. INTRODUCTION With the growing energy demand, more and more Li-ion batteries are needed in the portable electronics market and their territory has been expanded into large-scale electric energy storage such as in renewable power stations and hybrid electric vehicles because of their high specific capacity density and stable cycling capability.1−4 Currently, graphite-based carbon materials are widely used as the anode for Li-ion batteries due to their unique electrochemical and physical properties, despite their low theoretical specific capacity (372 mA h g−1).1 Recently, intensive researches have been carried out on exploring alternative anode materials with much higher capacities.2−4 Tin compounds such as tin metal, tin oxides, and tin sulfides with high theoretical gravimetric capacity and eco-friendliness are promising alternative anode materials.5−12 The major drawbacks associated with these materials are the rapid deterioration and low retention of capacity, mainly originating from their high volume changes during charge− discharge cycling.12 For resolving this issue, intensive attempts have been carried out on particle nanosizing, surface modification, and porous fabrication that provided higher surface area, offered more electron transport channels, and reduced strain associated with the lithium insertion process.6−8 On the basis of this consideration, 0D nanoparticles, 1D nanowires/nanotubes, 2D nanosheets, and 3D core−shell/ yolk−shell spheres of these electroactive Sn-based materials © 2016 American Chemical Society

have been designed and fabricated to improve cyclic performance and achieve high Li-ion storage.5−12 However, only a small number of studies have been carried out on tin sulfides compared to their oxides due to their lower microstructural flexibility.9−11 Comparing with the disulfide of SnS2, SnS shows a higher charge−discharge rate capability because of its intrinsic high electrical conductivity (0.193−0.0083 S·cm−1),13,14 indicating that SnS is a promising anode material. Similar with SnS2, the large volume change during Li+ intercalation and deintercalation from SnS results in breaking the integrity of the material and electrical contact between particles, thus resulting in rapid capacity decline upon extended cycling.9−11 The 2D hybrid sandwich-like nanosheet with a wellconductive component is a well-established microstructure for enhancing the electronic conductivity and Li+ diffusion kinetics of battery electrodes.15−18 This kind of unique microstructure can not only offer open channels for electron transport and Li+ion intercalation and deintercalation but also provide a stable structure framework.15 Furthermore, the Li-ion insertion in this kind of ultrathin nanosheets performs like surface Li-ion storage, which greatly reduces the Li-ion diffusion distance and thus achieves ultrafast charging/discharging.16 Herein, we have Received: January 17, 2016 Accepted: March 17, 2016 Published: March 17, 2016 8502

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

Research Article

ACS Applied Materials & Interfaces

Scheme 1. (a) Schematic Illustration of the Fabrication of Sandwich-like SnS/PPy Ultrathin Nanosheets from Uniform ZnSn(OH)6 Microcubes.a (b) Schematic Illustration of the Lithiation Process of These Sandwich-like SnS/PPy Ultrathin Nanosheets, Providing Sufficient Electron Conductive Paths and Electroactive SnS Volume Expansion

a

First, uniform ZnSn(OH)6 microcubes were facilely achieved by a room-temperature coprecipitation route. Subsequently, these uniform uniform ZnSn(OH)6 microcubes were hydrothermally treated with TAA and H4EDTA, in which Sn4+ was slowly released and reacted with S2−, forming ultrathin SnS2 nanosheets, while Zn2+ coordinated with H4EDTA, forming the soluble complex Zn-H2EDTA. Finally, these ultrathin SnS2 nanosheets were reduced into SnS by pyrrole, while pyrrole was in situ polymerized on the surface of nanosheets, resulting in sandwich-like SnS/PPy ultrathin nanosheets. with ethyl alcohol and distilled water, respectively, and finally dried at 50 °C under vacuum condition. 2.1.3. Synthesis of Sandwich-like SnS/PPy Ultrathin Nanosheets. Sandwich-like SnS/PPy nanosheets were synthesized via pyrroleinduce reductive transformation reaction in a hydrothermal environment. In a typical reduction and in situ polymerization procedure, 0.2 g of SnS2 ultrathin nanosheets was added to distilled water to form a yellow dispersion solution. Then, 0.8 mL of monomer pyrrole was added to the above dispersion solution. After being stirred for 30 min, the well-mixed dispersion was poured into a Teflon-lined autoclave, which was put in a constant temperature oven and maintained at 160 °C for 40 h. The samples were collected and washed with distilled water and anhydrous ethanol for three times, respectively, and finally dried at 50 °C under vacuum condition. 2.2. Materials Characterization. The composition of these collected products were characterized by X-ray diffractometry (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu Kα radiation source. Scanning electron microscopy (SEM) analysis was measured with a Zeiss Gemini DSM 982 scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses were performed with a JEOL 4000FX microscope. The Raman spectrum was conducted using a JOBIN YVON HR800 Confocal Raman system, and the Fourier transform infrared spectrum (FTIR) was performed on a Tensor-27. The PPy content in these finally achieved sandwichlike SnS/PPy composites was estimated with a thermogravimetric analysis (TGA) Q50 V20.8 Build 34 with an O2 flow rate of 10 mL min−1 and a heating rate of 10 °C min−1 from 30 to 800 °C. 2.3. Electrochemical Testing. The corresponding electrochemical performances of the as-prepared samples were tested by using Swagelok-type cells. The working electrodes were prepared by mixing 70 wt % sandwich-like SnS/PPy nanosheets, 20 wt % carbon black, and 10 wt % polyvinylidene fluoride (PVDF) in N-methy1-2-

successfully realized this strategy in the promising SnS-based anodes for Li-ion batteries. The sandwich-like polypyrrole/ SnS/polypyrrole ultrathin nanosheets (denoted as sandwichlike SnS/PPy ultrathin nanosheets) with large interfacial tensions were fabricated by a facile hydrothermal reduction and in situ polymerization route. The ultrathin 2D nanosheet structure of electroactive SnS facilitates Li+ diffusion into the bulk anode material, while the well-conductive PPy layer on both sides of the electroactive SnS ensures fast electron transport. Thus, these sandwich-like SnS/PPy ultrathin nanosheets exhibit outstanding electrochemical performance in terms of high specific capacity, long cycle life, and high rate capability.

2.. EXPERIMENTAL SECTION 2.1. Materials Synthesis. 2.1.1. Synthesis of Uniform ZnSn(OH)6 Microcubes. Uniform ZnSn(OH)6 microcubes were synthesized by a facile and low-cost coprecipitation route. In a typical synthesis process, 8 mmol of SnCl4 was added into 40 mL of ethanol to form solution A. ZnCl2 (8 mmol) and citric acid (8 mmol) were added into 280 mL of aqueous solution to form solution B. Then, solution A was added into solution B under stirring; after a few minutes, 40 mL of aqueous solution of NaOH (2 M) was quickly added into the above mixed solution at room temperature. After 1 h, the resulting white precipitate was centrifuged with distilled water and ethanol three times and dried under vacuum at 50 °C. 2.1.2. Synthesis of Ultrathin SnS2 Nanosheets. In a typical preparation procedure, 1 mmol of the above obtained ZnSn(OH)6 microcubes, 4.5 mmol of ethylenediaminetetraacetic acid (H4EDTA), and 5 mmol of thioacetamide (TAA) were hydrothermally treated at 180 °C for 12 h. The collected samples were then washed three times 8503

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

Research Article

ACS Applied Materials & Interfaces

Figure 1. SEM, TEM, and XRD characterizations of uniform ZnSn(OH)6 microcubes as the tin resource for slowly releasing Sn4+ during the subsequent sulfidation treatment: (a, b) Low- and high-magnification SEM images of ZnSn(OH)6 microcubes. (c) TEM image of a typical ZnSn(OH)6 microparticle showing the well-defined cubic morphology. (d) XRD pattern of the pure-phase and well-crystallized ZnSn(OH)6 microcubes. pyrrolidone (NMP) to make a slurry. The active material loading of the electrodes is about 1.5−2.0 mg/cm2. The electrolyte is an ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume) of 1 M LiPF6. For assembling Li-ion batteries, a Li foil was used as the counter electrode. The cycling and rate performance were evaluated by an Arbin MSTAT battery test system with the cutoff voltage of 0.01 V for discharge and 3.0 V for charge. Cyclic voltammetry (CV) curves were measured with a VoltaLab 80 electrochemical workstation. The electrochemical impedance spectroscopy (EIS) was also recorded by the same instrument over the frequency ranging from 100 kHz to 0.1 Hz.

Finally, sandwich-like SnS/PPy ultrathin nanosheets were converted from these dispersive SnS2 ultrathin nanosheeets via the pyrrole-assisted reductive reaction of the hydrothermal system. The main conversion mechanisms involve the reduction of high-valence state Sn(IV) to low-valence state Sn(II) and the in situ oxidative polymerization of monomer pyrrole to PPy, according to the follow chemical reaction:

3. RESULTS AND DISCUSSION Scheme 1a illustrates the detailed formation process of sandwich-like SnS/PPy ultrathin nanosheets. First, uniform perovskite-type hydroxide ZnSn(OH) 6 microcubes were facilely obtained by a room-temperature coprecipitation route (see the Supporting Information for details). The formation of these ZnSn(OH)6 microcubes was mainly determined by the crystallization habit of cubic phase materials. A subsequent hydrothermal treatment (including sulfidation and complexing) of these microcubes in the presence of thioacetamide (TAA) and ethylenediaminetetraacetic acid (H4EDTA) resulted in 2D ultrathin SnS2 nanosheets. During this hydrothermal chemical process, H4EDTA can extract Zn2+ ions from ZnSn(OH)6 microcubes and hold them tightly, forming a soluble complex Zn-H2EDTA with a formation constant, Kform = 1016.4.19 At the same time, Sn4+ ions were slowly released from these solid microcubes and reacted with S 2+ derived from TAA decomposition. The total chemical reactions of this hydrothermal treatment can be formulated as

As shown in Scheme 1b, such sandwich-like SnS/PPy ultrathin nanosheets not only facilitate fast electron transport and Li+ diffusion but also offer a stable and flexible framework for electroactive SnS volume expansion, thus exhibiting high performance for Li-ion storage. Figure 1a,b shows scanning electron microscopy (SEM) images of ZnSn(OH)6 particles at low and high magnifications as obtained from a room-temperature coprecipitation route. As clearly displayed in Figure 1b, these solid particles show a regular cubic shape and high monodispersivity, which is also supported by the corresponding transmission electron microscopy (TEM) image (Figure 1c). The phase structure of the ZnSn(OH)6 microcubes was characterized by X-ray powder diffraction (XRD) measurement. Figure 1d shows that all of the diffraction peaks can be well indexed to primitive cubic phase ZnSn(OH)6 (JCPDS No. 74-1825). These uniform ZnSn(OH)6 microcubes acted as a important reaction reagent in the following reaction system. For example, when reaction reagent used for the synthesis of SnS2 ultrathin nanosheets was directly employed (SnCl4 as the reaction reagent), instead of using preprepared ZnSn(OH)6 microcubes, while keeping other reaction parameters unchanged, the resultant products were not SnS2 ultrathin nanosheets but SnS2 micron-sized particles assembled from thick nanoplates (Figure S2, Supporting Information). During the crystallization process of SnS2 ultrathin nanosheets,

TAA + H 2O → H 2S + CH3COONH4

(1)

ZnSn(OH)6 + H4EDTA + 2H 2S → Zn(H 2EDTA) + SnS2 + 6H 2O

(2) 8504

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

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Figure 2. SEM and TEM measurements of ultrathin nanosheet-like SnS2 precursor: (a, b) Low- and high-magnification SEM images of ultrathin nanosheet-like SnS2 showing their curved 2D morphology. (c, d) Low- and high-magnification TEM images revealing that these 2D nanosheets have a smooth surface and average thickness of about 10−20 nm. (e) Typical HRTEM image showing these well-crystallized 2D SnS2 nanosheets with several layers. (f) The crystal structure of hexagonal SnS2 shown along the (010) axis view (atom color codes: red, tin; yellow, sulfur).

the releasing rate of Sn4+ ions from ZnSn(OH)6 plays a vital role. When the hydrothermal crystallization temperature was largely increased from 180 to 220 °C, only SnS2 thick nanosheet-assembled hierarchical microcubes (still preserving the cubic framework of ZnSn(OH)6 particle) were obtained (Figure S2, Supporting Information), while not dispersive or nonassembled nanosheets. It is worth noting that the strong coordinating ability of H4EDTA is crucial to completely accomplish the transformation of ZnSn(OH)6 to SnS2. When H4EDTA was substituted by tartaric acid, the synthetic products were ZnS phase and a small part of SnS2 (results not shown). This is caused by that tartaric acid has a relatively lower coordination ability with Zn2+ (Kform = 108.32),19 and thus results in weaker extracting and poorer binding ability to Zn2+ ions. Figure 2a is a low-magnification SEM image of the hydrothermally treated microcubes, which shows that these uniform ZnSn(OH)6 microcubes were completely transformed into 2D nanosheets after hydrothermal treatment. The local magnification SEM image (Figure 2b) clearly displays the ultrathin character of these 2D SnS2 nanosheets with a thickness of 5−10 nm. Furthermore, the representative transmission electron microscopy (TEM) image of as-transformed SnS2 ultrathin nanosheets is shown in Figure 2c. The

magnified TEM image (Figure 2d) of these ultrathin nanosheets clearly shows smooth surfaces free of particles. These 2D nanosheets are generally with width surfaces in the region of 200−500 nm. The high-resolution TEM (HRTEM) analysis (Figure 2e) further reveals the feature of the crystallographic structures of these nanosheets. Figure 2f clearly displays the side view of a 2D nanosheet with an interplanar crystal spacing of approximate 0.59 nm, corresponding to (001) planes of hexagonal phase SnS2. On the basis of this result, we can obtain that these dispersive SnS2 ultrathin nanosheets are normally composed of 10−15 layers stacking of the monatomic graphene-like 2D nanosheets. Figure 3a−c displays SEM images of the as-prepared SnS/ PPy products; they reveal that the 2D graphene-like nanosheets structure was undestroyed during the in situ polymerization process. The low-magnification TEM image (Figure 3d) shows that a veil-like microstructure with transparency can be distinguished, suggesting that the PPy polymer homogeneously coated on two sides of SnS nanosheets, forming sandwich-like nanosheets. The sandwich-like structure of PPy/SnS/PPy can be also supported by a typical broken composite nanosheet (Figure 3e). The HRTEM image (Figure 3f) indicates that the framework of the 2D nanosheets consists of a crystalline SnS 8505

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

Research Article

ACS Applied Materials & Interfaces

Figure 3. SEM and TEM measurements of sandwich-like SnS/PPy ultrathin nanosheets: (a−c) Low- and high-magnification SEM images showing that SnS/PPy composites preserve the SnS2 precursor’s 2D nanosheets-like morphology. (d, e) Low- and high-magnification TEM images of SnS/ PPy ultrathin nanosheets; the inset of (e) shows the schematic structure sandwich-like nanosheets. (f) HRTEM image showing that the pristine SnS2 smooth nanosheets were in situ reduced into porous SnS and fully encapsulated by conductive PPy layers.

component (the clear lattice spacing of about 0.34 nm, corresponding to the SnS(120) plane distance) and an amorphous PPy component. Dark-field TEM and the corresponding elemental mapping images of the unique sandwich-like SnS/PPy ultrathin nanosheets also well indicated that the conductive polymer PPy layers uniformly encapsulated the inner mesoporous SnS nanosheets (Figures S3 and S4, Supporting Information). To further determine the constitution and purity of these precursors and products, X-ray diffraction (XRD) was performed. The typical XRD pattern (Figure 4a) showed these 2D ultrathin nanosheets as the hexagonal SnS2 (JCPDS No. 23-0677). The XRD pattern (Figure 4a) also certainly confirmed full reduction of SnS2 by pyrrole, in which all the diffraction peaks of the as-converted products can be indexed to the orthorhombic phase of SnS (JCPDS No. 39-0354). In order to evaluate PPy polymer content in these SnS/PPy ultrathin nanosheets, thermogravimetric analysis (TGA) was performed. The TGA curve indicates that a weight loss of 2% up to 350 °C is related to

release of absorbed water in the composite, and the following 20% weight loss (350−800 °C) is believed to correspond to the decomposition and oxidation of PPy in air, resulting in the mass of the composite decreasing slowly. It is well-known that the SnS can be oxidized to SnO2 in the thermogravimetric process, based on complete oxidation of SnS. Therefore, through the TGA curves, we can clearly observe that the total weight loss is 22%, with 2% weight loss being adsorbed water, and thus calculate that the PPy content is about 20%. According to previous research work, 20 the detailed mechanisms of pyrrole oxidation and SnS2 reduction may be taken place according to the following steps: First, pyrrole monomers attacked the surface of SnS2 2D nanosheets and reduced the high-valence Sn4+ in the SnS2 frameworks to lowvalence Sn2+ at elevated hydrothermal temperature, resulting in the transformation of pristine SnS2 into SnS. Meanwhile, pyrrole monomers were oxidized to form conductive polymer PPy. As the reduction and polymerization reactions were taking place at the same time, the newly formed polymer PPy could 8506

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

Research Article

ACS Applied Materials & Interfaces

The Li-ion storage performance of the as-prepared sandwichlike SnS/PPy ultrathin nanosheets was fully studied by galvanostatic charge/discharge testing and cyclic voltammetry (CV) using two-electrode Swagelok-type cells. Figure 5a shows the first five CV curves of the SnS/PPy anode with the scanning rate of 0.1 mV s−1. The resulting CV profile of the SnS/PPy anode shows three obvious peaks during the initial cathodic scan. The peaks at 1.12 and 1.85 V in this image (Figure 5a) are associated with the formation of Li2S and decomposition of SnS (eq 4) that may happen in two steps, as reported by Kim and co-workers,29,30 whereas the peak < 0.15 V represents the Liion alloying with Sn (eq 5).9,10 In addition, the peak at 1.12 V that could be seen during the following cycles represents a partial reversibility behavior of the SnS/PPy anode. Previous researches on tin sulfide anodes have also shown that partial Li ions could intercalate into the layers of tin sulfides without causing phase decomposition.29,31 On the basis of these results, the decomposition of SnS (eq 4) may be divided into two procedures, as exhibited in the Supporting Information (eqs 1 and 2). SnS + 2Li+ + 2e− → Sn + Li 2S

(4)

Sn + 4.4Li+ + 4.4e− ↔ Li4.4Sn

(5)

Figure 5b exhibits the voltage−capacity curves of these sandwich-like SnS/PPy ultrathin nanosheets performed with a voltage region of 0.01−3.0 V at 0.1C (1C = 1000 mA g−1). As shown in this figure, the first discharge and charge capacities are 1828 and 1086 mA h g−1 (calculated based on SnS present in the sandwich-like nanocomposites), respectively. The initial Coulombic efficiency is approximate 59.4%, and such low first efficiency was mostly caused by partial reversibility decomposition behavior of SnS during the first discharge process and the consumption of Li ions and electrolyte for the formation of SEI film.32,33 In the following discharge steps, these sandwichlike SnS/PPy ultrathin nanosheets delivered a large reversible capacity of about 1000 mA h g−1, close to their theoretical capacity (about 1138 mA h g−1). These sandwich-like SnS/PPy ultrathin nanosheets exhibited a high cycling stability after the initial several cycles (Figure 5c). The sandwich-like SnS/PPy anode can remain on a high discharge capacity of 967 mA h g−1 after 50 cycles. In order to evaluate the long cycling performance, their cycling test at 1C for 500 cycles was also carried out. Over the whole charge−discharge processes, outstanding cycling performance is exhibited (except for the first 100 cycles with a slight fade), and a high specific capacity of about 703 mA h g−1 after long 500 cycles can be observed (Figure 5d). After another 200 charging/discharging cycles (at the 300th cycle), partial PPy encapsulated layers in these sandwich-like nanosheets may gradually become connected to each other (Figure S7, Supporting Information), further strengthening the current conductive and stable skeleton,35 resulting in the slight increase of capacity in the subsequent 200 cycles. Further cycling tests of these sandwich-like SnS/PPy ultrathin nanosheets at different rates (from 0.2C to 5C) showed that they also exhibited superior rate performance as anode materials for rechargeable Li-ion batteries (Figure 5e). Furthermore, at a high 5 A g−1 rate (after 40 cycles with the rate increased from 0.2C to 2C step by step), these sandwich-like SnS/PPy nanocomposites still maintained a high capacity of approximate 561 mA h g−1 (Figure 5e). As displayed in this figure, these sandwich-like nanosheets exhibit a high steady capacity value. The rate capability of these sandwich-like SnS/

Figure 4. (a) XRD patterns of the SnS2 ultrathin nanosheets and sandwich-like SnS/PPy nanosheets, clearly showing that the SnS2 (JCPDS No. 23-0677) precursor has been converted into SnS (JCPDS No. 39-0354) product completely. (b) TGA curve of sandwich-like SnS/PPy nanosheets showing approximately 20 wt % PPy in the SnS/ PPy nanocomposites.

fully cover on the as-transferring SnS through chemical interaction with the ligand amine group.20 After the chemical reduction, the SnS2 smooth single-crystalline nanosheets were changed into SnS porous polycrystalline nanosheets (Figure 3f). The formation of porous structures was mainly caused by the partial sulfur stripping,20 strain (due to lattice mismatch of orthorhombic SnS and hexagonal SnS2) release,20,21 and the byproduct H2S (eq 3) release.22 In addition, the molecular structure of PPy on SnS was studied by Raman spectroscopy (Figure S5, Supporting Information) and Fourier transform infrared spectroscopy (FTIR, Figure S6 in the Supporting Information) measurements. As clearly displayed in the Raman spectrum, all the peaks present in the Raman spectrum (1800− 800 cm−1) belong to the characteristic Raman bands of PPy (Table S1, Supporting Information). Bands located at 1051, 983, and 933 cm−1 can be attributed to C-H in-plane deformation of PPy, ring deformation, and C-H out-of-plane deformation, respectively.23,24 The band at 1600 cm−1 and the double bands at 1340 and 1410 cm−1 can be ascribed to CC backbone stretching and ring-stretching.25,26 The FTIR spectrum of sandwich-like SnS/PPy ultrathin nanosheets also exhibited the characteristic absorption bands of PPy (Figure S6, Supporting Information). The characteristic peaks (Table S2, Supporting Information) appear at 1570, 1505, and 1400 cm−1 (the intrinsic vibrations of the pyrrole rings), 1198 cm−1 (the vibration of C-N stretching), 1300 and 1045 cm−1 (the in-plane vibration of C-H), and 1115 cm−1 (the deformation of NH+ inplane vibration, formed on the polymer chains by protonation).27,28 8507

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

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ACS Applied Materials & Interfaces

Figure 5. Li-ion storage performance of sandwich-like SnS/PPy ultrathin nanosheets: (a) CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01−3.0 V. (b) Voltage−capacity curves at 0.1C. (c) Cycling performances of sandwich-like SnS/PPy ultrathin nanosheets at 0.1C. (d) Long cycling performances of sandwich-like SnS/PPy nanosheets at 2C. (e) Rate capability at different rates (increased from 0.2C to 5C). (f) Rate comparison of rate capability of sandwich-like SnS/PPy ultrathin nanosheets with all other recently reported SnS-based anodes for Li-ion batteries.

All of these microstructural features endow the current tin sulfide anode with high cycling stability and superior rate capability.

PPy ultrathin nanosheets has also been compared with those of recently reported SnS-based anodes for Li-ion batteries, including bare SnS nanobelts30 and hierarchical hollow microspheres,34 SnS/PPy nanobelts,35 SnS/graphene nanocomposites,36 SnS/CNT nanocomposites,37 and the just recently reported 3D porous SnS/C films.38 It can be clearly observed that the rate performance of the current sandwich-like SnS/PPy ultrathin nanosheets is the best among all of these recently reported SnS-based anodes for Li-ion batteries. The structure stability was substantiated by the observed SEM images of the SnS/PPy anode after cycling (Figure S8, Supporting Information), with the 2D nanosheet shape being well-preserved. This result suggests that the soft conducting PPy polymer with good mechanical flexibility and chemical stability offers enough space for the volumetric expansion of electroactive SnS during lithiation and delithiation processes, which is much similar to the function of the graphene matrix in graphene-encapsulated electrodes.39−41 Moreover, the unique sandwiched framework surely facilitates the necessary electron and Li-ion diffusion (Figures S9−S12, Supporting Information), and the inner encapsulated SnS ultrathin nanosheets can effectively reduce the diffusion distance of electrons and Li ions.

4. CONCLUSION In summary, we have successfully developed a facile route to synthesize stable sandwich-like SnS/PPy composites for Li-ion batteries, in which the 2D ultrathin nanostructure facilitates Li+ insertion and PPy greatly improves the electronic conductivity for fast electron supply. The sandwich-like SnS/PPy nanosheets exhibit a high specific capacity of about 1000 mA h g−1 at 0.1C, superior cycling stability (703 mA h g−1 after 500 cycles at 1C), and remarkable rate capability (reversible capabilities of 878, 805, 747, 652, and 576 mA h g−1 at 0.2C, 0.5C, 1C, 2C, and 5C, respectively). We believe that this facile synthesis route can be applied to other multivalence metal oxides and sulfides for highperformance Li-ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00627. 8508

DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

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ACS Applied Materials & Interfaces

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SEM images of SnS2 nanosheet aggregates and SnS2 nanosheet-assembled hierarchical microcubes, Raman and FTIR spectra of sandwich-like SnS/PPy nanosheets, BET and elemental mapping results of sandwich-like SnS/PPy nanosheets, SEM images of sandwich-like SnS/ PPy nanosheets after 50 and 500 cycles, EIS spectra and Li-ion storage performances of sandwich-like SnS/PPy ultrathin nanosheets and bare SnS2 ultrathin nanosheets (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Projects 11202177, 51271078, and 51231003).

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DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510

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DOI: 10.1021/acsami.6b00627 ACS Appl. Mater. Interfaces 2016, 8, 8502−8510