High-rate performance of SnS2 nanoplates without carbon-coating as anode material for lithium ion batteries

Electrochimica Acta 112 (2013) 439–447

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High-rate performance of SnS2 nanoplates without carbon-coating as anode material for lithium ion batteries Lingyan Wang a,b,c , Linhai Zhuo a,b,∗ , Yancun Yu a,b , Fengyu Zhao a,b,∗ a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, 130022 Jilin, China b Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China c University of the Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 31 May 2013 Received in revised form 30 July 2013 Accepted 26 August 2013 Available online 6 September 2013 Keywords: SnS2 nanoplates Temperature Forming process Lithium ion battery

a b s t r a c t Two-dimensional (2D) SnS2 nanoplates were synthesized through a facile hydrothermal method. The influences of reaction conditions such as temperature and pH on the size, crystallinity and the forming process of SnS2 were investigated in detail. At low temperature (160 ◦ C), the SnS2 nanoplates showed poor crystallinity; while at higher temperatures above 200 ◦ C, the crystallinity and thickness of the SnS2 nanoplates tended to increase. In addition, pH had notable impact on the nucleation velocity of SnO2 and the conversion speed from SnO2 to SnS2 in further as well. When used as anode materials in rechargeable lithium ion batteries, the SnS2 nanoplates synthesized at 200 ◦ C and pH = 10.5 (SnS2 -200-10.5) showed the best lithium storage capacity, good cycling stability and excellent rate capability. It retained a high reversible capacity of 521 mA h g−1 over 50 cycles at a current of 100 mA g−1 , equal to 90.0% of the initial reversible capacity. In addition, the coulombic efficiency increased from 36% in the first cycle to over 97% in the subsequent cycles. Even at high current densities of 1, 2 and 3 A g−1 , the electrodes could still delivery as high as 472, 397 and 340 mA h g−1 , respectively. The enhanced electrochemical performance of the SnS2 -200-10.5 can be attributed to the compact and regular crystal structure with a moderate thickness and crystallinity, which is beneficial for maintaining the stability of the structure and fast ion transport during lithiation/delithiation processes. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction With the ever-increasing requirements of electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium ion batteries (LIBs) possessing larger energy density, higher power density, longer cycle life and high safety are urgent needs [1]. However, the relatively low storage capacity (372 mA h g−1 ) of commercially used graphite carbon still restricts its application in LIBs with high energy and power densities [2]. The tremendous challenge has motivated intense research interest aiming at alternative electrode materials with higher capacity and cycling stability for next-generation LIBs. Recently, various Sn-based materials such as SnO2 [3–10] and SnS2 have been studied as possible candidates to carbonaceous anode materials for LIBs since they were demonstrated to have improved lithium storage capacity over graphite carbon. Similar to SnO2 , the lithium storage in SnS2 relies on the reversible alloying/de-alloying

∗ Corresponding authors. Tel.: +86 431 85262410; fax: +86 431 85262410. E-mail addresses: [email protected] (L. Zhuo), [email protected] (F. Zhao). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.08.154

reaction between lithium and metallic Sn nanocrystals which are generated from the initial reduction of SnS2 . Both of them uniformly produce an inert matrix such as Li2 O produced from SnO2 [11–13] and Li2 S produced from SnS2 [13–15], which can not only buffer the volume change during cycling but also prevent the aggregation of active materials, thus leading to improved cycling stability. Compared with SnO2 crystallized in a tetragonal rutile-type structure, SnS2 has a layered hexagonal CdI2 -type crystal structure, in which each layer of Sn atoms are sandwiched between two layers of hexagonally close-packed S atoms, and the adjacent sulfur layers are connected by the weak van der Waals interaction [14,16]. This crystallographic feature is suitable for the intercalation of lithium ions and the compensation of the lithiation/delithiation volume change [17–20]. Based on such crystal structure, SnS2 can be easily cleaved to allow the generation of atomically smooth surfaces along the (0 0 1) basal plane [21]. To date, extensive efforts have been devoted to synthesize various SnS2 with an improved electrochemical performance as alternative anode materials. Unfortunately, the capacity fading of SnS2 electrode materials still exists due to large volume changes during the electrochemical alloy formation. To circumvent the

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problem, on one hand, more and more attention has been paid to make various SnS2 nanostructures with different morphologies, such as 3D-hierarchical structures [1], hollow sphere [22], flower-like [23], micro/nano-structures [24], novel nanocrystalline morphologies [25]. However, almost all these preparations of SnS2 nanomaterials require either special device, toxic reagents, surfactants or special reaction conditions such as a relatively high temperature, free of water and oxygen, etc. Therefore, simple, inexpensive and environment-friendly methods for the synthesis of SnS2 with high electrochemical performance have drawn much attention in recent years. On the other hand, a great proportion of research focuses on improving the conductivity of SnS2 by combining the electronically conductive agents, such as C [26,27], CNTs [28] or graphene [29–31]. For example, Zhang and his coworkers remarkably enhanced the lithium storage performance of SnS2 nanoplates through carbonization of organic surfactants (oleylamine and oleic acid) at 700 ◦ C [27]. Zhi et al. successfully synthesized SnS2 -graphene hybrids by transforming tin oxide nanoparticles into tin disulfide nanoplates directly on/between graphene nanosheets via a solution approach followed with a simple chemical vapor deposition (CVD) process, which enhanced lithium storage properties with high reversible capacity and good cycling performance [29]. More recently, our group reported one-step hydrothermal method to synthesize SnS2 /graphene composites and found that the as-synthesized electrode exhibited a significantly enhanced cycling performance with excellent capacity retention of 96% after 30 cycles [30]. Besides, Chen et al. adopted a facial process for preparing few-layer SnS2 /graphene (FL-SnS2 /G) hybrid by a facile l-cysteine-assisted solution-chemistry method. They confirmed that the incorporation of graphene could improve cycling stability and rate capability considerably [31]. Although there are a lot of reports described the hydrothermal synthesis of SnS2 , the detailed insight into the influences of reaction conditions such as temperature and pH on the morphology and structure of SnS2 formed as well as its electrochemical performance, especially rate performance has remained scarce. Herein, we synthesized the SnS2 nanoplates with using SnCl4 ·5H2 O and thioacetamide (TAA) as the starting materials, and the effects of temperature and pH on the crystallinity and the forming process of SnS2 were studied as well. We found the temperature and pH value had significant influences on the crystallinity and nucleation rate during SnS2 formation, and so the SnS2 were prepared by carefully controlling the growing of SnS2 crystal grains via adjusting the temperature and pH value. The electrode assembled by SnS2 (200 ◦ C; pH, 10.5), obtained by the optimal condition, exhibited the best lithium storage capacity, good cycling stability (521 mA h g−1 , 90%) and excellent rate performance (472, 397 and 340 mA h g−1 , at the current densities of 1, 2 and 3 A g−1 ).

2. Experiment 2.1. Materials and synthesis of SnS2 nanoplates All reagents were analytical grade and used directly without further purification. Tin (IV) chloride pentahydrate (SnCl4 ·5H2 O) and sodium hydroxide (NaOH) were purchased from Beijing Chemical Works. TAA was purchased from Aladdin Chemistry Co. Ltd. Deionized water was used for all manipulation. SnS2 nanoplates were synthesized from SnCl4 ·5H2 O and TAA via a hydrothermal route. In a typical experiment, 0.75 mmol SnCl4 ·5H2 O and 6 mmol TAA were dissolved in 30 ml deionized water to form a transparent solution. After stirring for 1 h, the pH value was adjusted to appropriate value from 0 to 10.5 using 0.5 M NaOH solution and stirred for another 30 min. Then, the solution was transferred to a Teflon-lined stainless steel autoclave of

50 ml capacity and heated in an electric oven at different temperatures starting from 160 to 240 ◦ C with an interval of 40 ◦ C for 24 h. After cooling naturally, a yellow precipitate was collected by centrifugation, washed with deionized water and then ethanol several times, and dried in vacuum at 80 ◦ C for 12 h. In order to facilitate the later statement, the SnS2 products synthesized under different temperatures and pHs were hereinafter denoted as SnS2 -T-pH, e.g., SnS2 -200-10.5 standing for the SnS2 sample synthesized at 200 ◦ C with pH of 10.5. 2.2. Characterizations The crystallographic structure of as-prepared samples was characterized with X-ray power diffraction [XRD, Bruker D8 ˚ Advance diffractometer using Cu K␣ ( = 1.5418 A)]. The morphology of the materials was analyzed by the Field Emission scanning electron microscope (FE-SEM Hitachi S-4800) equipped with energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) were recorded on a Tecnai G20 operating at 200 kV for the detailed microstructure information of the samples. 2.3. Electrochemical measurements The electrochemical tests were measured using two-electrode 2025-type coin cells assembled in an argon-filled glove box. Lithium sheets served as the counter electrode and reference electrode, and a polypropylene film (Celgard-2300) was used as a separator. The electrolyte was a 1.0 M LiPF6 solution in a mixture of EC-DMC (1:1 in volume). The working electrode was prepared by mixing SnS2 as the active material, acetylene black (AB) as the conducting material and polyvinylidene fluoride (PVDF) as the binder in a weight ratio of 80:10:10, dissolved in 1-methyl-2pyrrolidinone (NMP) to form slurry. Then the slurry was spread on a copper foil, which acted as a current collector, and dried at 120 ◦ C overnight in vacuum before transferring into an argon-filled glove box. Galvanostatic charge/discharge cycles were carried out on a battery tester between 0.005 and 1.2 V vs. Li/Li+ at various current densities on a NEWARE cell test instrument (Shenzhen Neware Electronic Co., China). Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) measurements were performed by using a VMP3 Electrochemical Workstation (Bio-logic Inc.). 3. Results and discussion 3.1. Analysis of structure and morphology The crystallographic structure and phase purity of the samples obtained were studied by XRD. Fig. 1 shows the diffraction profile of SnS2 -200-10.5 with a calculated lattice parameter of ˚ which can be indexed to a = 3.162 ± 0.003 A˚ and c = 5.890 ± 0.003 A, crystalline hexagonal SnS2 (JCPDS Card No. 23-0677). The thickness of SnS2 nanoplates was estimated as ca. 13 nm according to the (0 0 1) peak by the Scherrer equation. No additional diffraction peak is observed, implying a high purity of the final product. Fig. 2a and b present typical SEM images of the SnS2 -20010.5. It’s clear that the as-prepared SnS2 products are mainly nanoplates with a confined hexagonal symmetry and a lateral size of 100–150 nm. A side view of SnS2 nanoplates reveals a thickness of 13 nm, which is complying with the estimated thickness of XRD. Besides, TEM image further demonstrates the hexagonal morphology, as shown in Fig. 2c. A more revealing feature of the crystallographic structure of the 2D SnS2 nanoplates is confirmed by the HRTEM analysis (Fig. 2d). Specially, the SnS2 nanoplate lying flat on the copper grid with the zone axis of [0 0 1] has aligned

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its single crystalline nature. Based on above results, it can be concluded that SnS2 nanoplates are in single crystalline structure and have 2D layered structure.

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lattice fringes with interplanar distance of 0.31 nm, which is consistent with the (1 0 0) planes of the hexagonal SnS2 phase. The fast fourier transform (FFT) image (Fig. 2d inset) of the nanoplate shows the characteristic spots for a hexagonal structure with the inner six spots indicating reflection of the (1 0 0) planes, further indicating

In order to investigate the influences of different conditions on the morphology of the products, a succession of parallel experiments were carried out by altering the parameters such as reaction temperature and pH value. Fig. 3 shows the XRD profiles of a series of SnS2 samples prepared under variant temperatures with a fixed pH of 10.5. It can be clearly seen that all of the SnS2 samples obtained at diverse temperatures are well indexed to the 2T-type hexagonal SnS2 . As expected, the diffraction peaks become narrower and sharper with the raising of temperatures in Fig. 3a. At 160 ◦ C, the diffraction peaks are relatively wide; while at temperature higher than 200 ◦ C, the diffraction peaks including (0 0 1), (1 0 0) and (1 1 0) get narrower and sharper, which indicates the SnS2 formed at temperature above 200 ◦ C has better crystalline phase compared to the sample formed at 160 ◦ C. SEM images (Fig. 3b–d) prove that all of the obtained samples exhibit the morphology of nanoplates with a lateral size ranging at ca. 100–150 nm. With the increase of temperature, however, there is a visible increase in the average thickness of nanoplates. For instance,

Fig. 2. Morphological characterization of the SnS2 -200-10.5 nanoplates: (a) planar SEM image, (b) lateral SEM image, (c) TEM image, (d) HRTEM image. The inset is the corresponding fast Fourier transform (FFT) pattern.

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Fig. 3. (a) XRD patterns of SnS2 nanoplates synthesized under different temperatures and the corresponding SEM images of (b) SnS2 -160-10.5, (c) SnS2 -200-10.5 and (d) SnS2 -240-10.5.

the thickness of nanoplates estimated is about 8 nm at 160 ◦ C, and it adds up to ca. 13 nm at 200 ◦ C and 30 nm at 240 ◦ C.

3.3. The influence of pH value In order to elucidate the detail forming process of the SnS2 nanoplates at different pH values, time-dependent experiments were carried out at pH = 0, 6.5 and 10.5, respectively. The SEM images (Fig. S1) confirm the as-prepared SnS2 samples synthesized at different pH values and 200 ◦ C mainly consist of nanoplates. Although there is no obvious difference in the final crystallinity of the SnS2 from Fig. 4a, the concrete formation processes still make a great difference of the samples synthesized under different pHs at early stage. Fig. 4b–d are the corresponding XRD patterns of the obtained products during different reaction periods. All of them involve the structural transformation process with the tetragonal SnO2 phase (JCPDS Card No. 41-1445) as an intermediate product. But in detail, SnO2 phase begins to appear at 10, 0 and 30 min for SnS2 -200-0, SnS2 -200-6.5 and SnS2 -200-10.5, respectively. Along with the extension of reaction time, SnO2 begins to transform into SnS2 , which can be detected at 30, 50 and 45 min for SnS2 -2000, SnS2 -200-6.5 and SnS2 -200-10.5, respectively. In addition, the contents of surface oxygen, sulfur and tin of SnS2 -200-6.5 reacting for 40 and 50 min were determined by SEM-EDX analysis (Fig. S2 and Table S1). The EDX analyses show that the sample is primarily made up of oxygen and tin elements at 40 min with a trace amount of sulfur which may be absorbed on the surface. With prolonging reaction time to 50 min, the content of oxygen reduces; on the contrary, the atomic ratio of sulfur increases obviously (Table S1), which is consistent with the result of XRD (Fig. 4c). On the basis of the above XRD results, we speculate that the formation of SnS2 nanoplates should contain the following reactions

as shown in Eqs. (1)–(7). At first, hydrolysis of Sn4+ occurs to form Sn(OH)4 white precipitate and then it transforms to SnO2 , which are more quick in acid and neutral solutions (Eqs. (1)–(2)). However, the precipitated Sn(OH)4 in alkaline solution can redissolve and transform to Sn(OH)6 2− with the existence of superfluous OH− (Eq. (3)), which leads to form SnO2 slowly. In another speaking, the nucleation rate of SnO2 is faster under acid and neutral conditions than that of alkaline condition. At the same time, TAA decomposes to generate H2 S in an acidic solution or S2− in an alkaline solution (Eqs. (4)–(5)), and the decomposition velocity is faster in acidic (pH = 0) or alkaline solution (pH = 10.5) than that in neutral surroundings (pH = 6.5). Finally, the generated H2 S or S2− reacts with SnO2 to form SnS2 (Eqs. (6)–(7)). According to the time-dependent experiments (Fig. 4), the transformation of SnO2 to SnS2 is very quick, and the overall execution takes 10–15 min approximately. While the last step of crystallization will take a long time under hydrothermal conditions. Sn4+ + 4OH− → Sn(OH)4 ↓

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SnO2 + 2H2 S → SnS2 + 2H2 O

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SnO2 + 2S2− + 2H2 O → SnS2 + 4OH−

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L. Wang et al. / Electrochimica Acta 112 (2013) 439–447

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Fig. 4. XRD patterns of the as-synthesized products at different pH values with the fixed temperature of 200 ◦ C: (a) the whole final patterns of 24 h for SnS2 and different reaction periods of (b) SnS2 -200-0, (c) SnS2 -200-6.5 and (d) SnS2 -200-10.5.

3.4. Electrochemical performances

known to represent the redox peak couple of the reaction of Li ions and Sn metal [13]. Such a redox peak couple is frequently observed in a similar potential range in the following cycles, indicating that Li ions subsequently reacted with Sn metal to form Lix Sn alloy as well as the reverse reaction. The additional peaks at 1.5 and 1.8 V can be attributed to lithium intercalation into the SnS2 layers without causing phase decomposition [32,33]. The additional oxidation peak at around 1.8 V observed in all of the anodic potential scans can be assigned to the lithium deintercalation from SnS2 layers without phase decomposition [27,30]. Additionally, it is different from most reported literature that our SnS2 -200-10.5 nanoplates electrode still shows one evident broad peak at ca. 1.4 V during the following cathodic sweeps, which indicates that the reaction is partially reversible [32,33].

In order to investigate the lithium storage property of the SnS2 nanoplates, a series of electrochemical measurements were carried out based on the half-cell configuration. Fig. 5a shows the cyclic voltammograms (CVs) curves of SnS2 -200-10.5 for the first three cycles from 0.01 to 2.5 V vs. Li/Li+ at the scan rate of 0.1 mV s−1 . Four obvious broad peaks are present at about 1.8, 1.5, 1.2 and 0.1 V in the first cathodic potential sweep. The reduction peak at 1.2 V can be ascribed to the decomposition of the SnS2 into metallic Sn and Li2 S as well as the formation of solid electrolyte interface (SEI), which is responsible for the irreversible capacity of SnS2 -based anodes in the first charge/discharge cycle. The more cathodic potential at around 0.1 V and the anodic potential at about 0.6 V in the first scan are

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Fig. 5. (a) Cyclic voltammograms and (b) 1st, 2nd, 25th and 50th galvanostatic charge–discharge voltage profiles of SnS2 -200-10.5 electrodes at 100 mA g−1 .

L. Wang et al. / Electrochimica Acta 112 (2013) 439–447

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Fig. 6a shows the typical results of cycling of SnS2 nanoplates anodes synthesized at various temperatures with a fixed pH of 10.5 cycled between 0.005 and 1.2 V at 100 mA g−1 . They give a similar initial irreversible discharge capacity of ca. 1440 mA h g−1 , while they show a distinct first reversible capacity of 459, 517 and 560 mA h g−1 for SnS2 -160-10.5, SnS2 -200-10.5 and SnS2 -24010.5, respectively. In addition, the discharge capacities of SnS2 anodes from 160 to 240 ◦ C are 433, 521 and 543 mA h g−1 after 50 cycles, and the capacity retentions are 84.1%, 90.0% and 88.7%, compared with the obtained 2nd discharge capacities (515, 579 and 612 mA h g−1 ), respectively. Fig. 6b displays the typical result of cycling performances of SnS2 anodes synthesized under different pH conditions at 200 ◦ C. The 2nd discharge capacities for SnS2 -200-0, SnS2 -200-2.5, SnS2 -200-6.5 and SnS2 -200-10.5 are 539, 501, 653 and 579 mA h g−1 , respectively, and they gradually reduce to 391, 327, 536 and 521 mA h g−1 after 50 cycles, corresponding to 72.5%, 65.3%, 82.1% and 90.0% capacity retentions. Fig. 6c and d show the rate performances of the as-prepared SnS2 samples, which were carried out at different current densities from 50 mA g−1 (0.08 C) to 3 A g−1 (4.65 C). As observed in Fig. 6c, SnS2 160-10.5 shows the worst rate performance, especially at high rate density. The average discharge capacities of the SnS2 anodes are maintained at 499, 459, 413, 372, 334, 291, 150 and 67 mA h g−1 from 50 mA g−1 to 3 A g−1 , respectively. In contrast, SnS2 -200-10.5 and SnS2 -240-10.5 manifest similar rate performances at lower current densities (from 50 mA g−1 to 1 A g−1 ). However, surprising discrepancies are manifested at higher current densities from 2 A g−1 (3.10 C) to 3 A g−1 (4.65 C). For example, SnS2 -240-10.5 has specific capacities of 350 and 274 mA h g−1 at current densities of 2 and 3 A g−1 , respectively. Nevertheless, for SnS2 -200-10.5, it shows outstanding and fancy electrochemical performances such

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Fig. 5b exhibits the galvanostatic charge/discharge voltage profiles of SnS2 -200-10.5 over the potential range of 0.005–1.20 V at a current density of 100 mA g−1 . Obviously, there are four plateaus at approximately 1.8, 1.6–1.5, 1.2 and 0.5–0.01 V present in the first discharge process, which correspond to the cathodic peaks in the CVs. During the first charge process, it shows only one potential plateau at about 0.6 V since the cutoff charge voltage is set up to 1.20 V. These agree well with the results of the CVs. In addition, the insertion process gives an initial discharge capacity of 1438 mA h g−1 , which is much larger than the theoretical first discharge capacity (1232 mA h g−1 ), i.e., the sum of the irreversible and reversible capacities of SnS2 [13,32]. The excessive initial discharge capacity is likely due to the side reaction of the electrode with electrolyte (forming SEI films) at the low voltage region (below 1 V), leading to a large amount of irreversibly trapped lithium [12,31]. The second cycle is highly reversible, giving the discharge and charge capacities of 579 and 540 mA h g−1 , respectively. After 50 cycles, the SnS2 -200-10.5 nanoplates electrode presents an excellent reversible capacity of 521 mA h g−1 , corresponding to 90.0% of the initial reversible capacity (Fig. 5b). To the best of our knowledge, the obtained 50st discharge capacity and the capacity retention (521 mA h g−1 , 90.0% at 100 mA g−1 ) of our SnS2 nanoplates exhibit superior value over some of the previously reported SnS2 -based anodes, e.g., graphene-SnS2 nanocomposites (388 mA h g−1 , 90.5% at 200 mA g−1 ) [34], flower-like SnS2 (387 mA h g−1 , ca. 75% at 100 mA g−1 ) [23], and they are comparable to 3D nanoflake SnS2 based hollow microspheres (532 mA h g−1 , 90% at 0.1 C) [22] and SnS2 nanoplates (513 mA h g−1 , 96% at 100 mA g−1 ) [19]. In view of the discrepancies of crystallinity and thickness, we tested the lithium storage properties of these as-prepared SnS2 nanoplates as anode materials for LIBs to evaluate the influences of temperature and pH on the electrochemical performance.

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Fig. 6. The anode performances of as-prepared SnS2 nanoplates at various conditions: (a), (b) cycle performances and (c), (d) rate performances from 50 mA g−1 to 3 A g−1 in the potential range of 0.005–1.2 V.

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as 397 mA h g−1 capacity at 2 A g−1 and 340 mA h g−1 capacity at 3 A g−1 . It is clear that SnS2 -200-10.5 exhibits excellent rate performance, which is much better than the other samples. Fig. 6d shows the rate cycling behavior of the as-prepared SnS2 samples under various pH conditions. The SnS2 -200-10.5 nanoplates anode, determined by our optimal experiment, has the best lithium storage capacity, good cycling stability and excellent rate performance. The capacity is stable at 588 mA h g−1 after 5 cycles at a rate of

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100 mA g−1 . Upon increasing the charge/discharge rates to 200, 400, 600, 800, 1000 and 2000 mA g−1 , its average discharge capacities are maintained at 564, 532, 510, 491, 472, and 397 mA h g−1 , respectively. Even at a very high current density, 3 A g−1 , SnS2 -20010.5 can still deliver an average discharge capacity of 340 mA h g−1 , which is much higher than that of SnS2 -200-0 (211 mA h g−1 ), SnS2 -200-2.5 (136 mA h g−1 ) or SnS2 -200-6.5 (208 mA h g−1 ). Even though the current changes from 3 A g−1 to 100 mA g−1 , the specific

Fig. 7. Characterizations of SEM, TEM and XRD (e) of SnS2 nanoplates anodes ran after 50 cycles at a current of 100 mA g−1 : (a), (b) SnS2 -200-10.5; (c), (d) SnS2 -200-2.5.

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capacity of the SnS2 -200-10.5 electrode finally reversibly return to ca. 560 mA h g−1 and changes very slightly in the following cycles, which shows significant superiority over other pH values. Consequently, the cycling and rate performances are greatly related to the crystallinity and architecture of the nanoplates as well. For the SnS2 -T-10.5 samples, the higher crystallinity and the thicker structure are, the better cycling performance is; while for the rate performance, since the thicker architecture is not conductive to fast ion transport during lithiation/delithiation processes, the SnS2 -200-10.5 nanoplates with the same crystallinity but thinner thickness than the SnS2 -240-10.5 exhibit better rate performance, especially at higher current densities. On the other hand, for the SnS2 -200-pHs, though they possess almost the same crystallinity and thickness from the XRD result, their electrochemical performances show a distinct difference, which are influenced by the nucleation rate of SnO2 and the transformation speed of SnO2 to SnS2 . The results prove that the relatively slow nucleation rate of SnO2 and the medium transformation speed of SnO2 to SnS2 are the prerequisites for obtaining excellent lithium storage capacity. To further discuss the distinct electrochemical performances of SnS2 , the SnS2 -200-10.5 and SnS2 -200-2.5 cells ran after 50 cycles at a current of 100 mA g−1 were opened in a glove box. As observed from the SEM images (Fig. 7a and c), the original composite structure of SnS2 -200-10.5 shows to basically retain its original composite structure, but the cycled SnS2 -200-2.5 is destroyed and crushed severely in its platy structure, resulting in loose-packed structure. In addition, the TEM observation (Fig. 7b) shows that the cycled SnS2 -200-10.5 dispersed uniformly in the matrix with size of about 10 nm, but the cycled SnS2 -200-2.5 agglomerated seriously (Fig. 7d). Moreover, it was also demonstrated by the XRD results as shown in Fig. 7e. The cycled SnS2 -200-10.5 shows characteristic diffraction peaks of Sn (JCPDS Card No. 04-0673) with excellent crystallinity, while the cycled SnS2 -200-2.5 does not give the complete diffraction peaks of Sn. The results indicate that the cycled SnS2 -200-2.5 electrode pulverized and crushed in the microstructure of the active materials, which in turn leads to fast capacity fading and inferior cycling stability since void space between the nanoparticles is greatly substantial to accommodate the volume change for anodes [17,18,35]. On the contrary, the cycled SnS2 200-10.5 still preserves the pristine morphology and uniformly distribution without agglomeration, which benefits for the fast ion transport/electrolyte infiltration and provides space for accommodating the volume expansion of SnS2 during lithiation/delithiation processes, thus resulting in a better cycling performance. Accordingly, associated with the SEM images (Fig. S1), it is speculated that the relatively slow nucleation rate of SnO2 and the medium transformation speed of SnO2 to SnS2 under alkaline surroundings should contribute to the formation of more compact and regular crystal structure, which may be one of the prime reasons for the improved electrochemical performance. To further explain and better understand the reasons for the excellent rate capability of SnS2 -200-10.5, electrochemical impedance spectroscopy (EIS) measurements were carried out on the SnS2 -200-2.5 and SnS2 -200-10.5 cells cycling after 50 cycles using a sine wave of 5 mV amplitude in the frequency range from 0.1 Hz to 100 kHz. As shown in Fig. 8, both of the impedance curves are composed of one compressed semicircle in the medium frequency range, which could be assigned to charge-transfer resistance (Rct ) on electrode/electrolyte interface, and an approximately 45◦ inclined line in the low frequency range, which represents the Warburg impedance (W) associated with Li-ion diffusion into the bulk of the electrode material [29]. Moreover, the intercept at the Zre axis at high frequency refers to Rs , which includes electrolyte solution resistance, electric contacts resistance, and ion conductive resistance. The Rs and Rct values are calculated with ZsimpWin

300 SnS2-200-2.5 SnS2-200-10.5

250

- Zim(Ω)

446

200 150 100 50 0 0

50

100

150

200

250

300

Zre(Ω) Fig. 8. Nyquist plots of the SnS2 -200-2.5 and SnS2 -200-10.5 electrodes after the 50th cycle at a rate of 100 mA g−1 obtained by applying a sine wave with an amplitude of 5.0 mV in the frequency range from 100 mHz to 100 kHz.

software, which are 8  and 23  cm−2 for the SnS2 -200-10.5, 14  and 166  cm−2 for the SnS2 -200-2.5 electrode, respectively. It is apparent that the SnS2 -200-10.5 composite electrode has smaller resistance than that of the SnS2 -200-2.5, indicating that the former possesses higher electronic conductivity and faster chargetransfer reaction for lithium ion insertion and extraction, which may account for the excellent performance of the SnS2 -200-10.5. Such excellent rate performance is superior to some bare SnS2 and comparable to some carbon-based SnS2 materials reported, as summarized in Table S2. The best sample of SnS2 -20010.5 in this work shows a capacity of 472 mA h g−1 at 1 A g−1 , which is much higher than some reported ones for bare SnS2 , they are, SnS2 nanobelts (273 mA h g−1 ) [36], SnS2 –SiO2 NRs (400 mA h g−1 ) [37] and SnS2 -SnO2 (370 mA h g−1 ) [38]. Besides, its capacity (510 mA h g−1 ) at 600 mA g−1 is comparable to SnS2 array (514 mA h g−1 , at 0.9 C) and its capacity (491 mA h g−1 ) at 800 mA g−1 is better than SnS2 array (460 mA h g−1 , at 1.2 C) [39]. Furthermore, its capacity at lower current of 200 mA g−1 is 564 mA h g−1 , it is better than SnS2 @MWCNT (469.9 mA h g−1 ) [28] and comparable to the data (650 mA h g−1 ) of SnS2 /GNS [29] and SnS2 /C [26]. 4. Conclusions In summary, 2D hexagonal SnS2 nanoplates were synthesized through a facile one-pot hydrothermal method. The effect of reaction temperature and pH on the size, crystallinity and the detail formation process of SnS2 nanoplates were studied in detail. The results show that the temperature has significant effect on the crystallinity and the thickness of the SnS2 nanoplates; while the pH could further affect the internal crystal structure through adjusting the nucleation rate of SnO2 and the transformation speed of SnO2 to SnS2 , and thus affect their electrochemical performances. The SnS2 -200-10.5 electrode shows the best lithium storage capacity, good cycling stability and excellent rate capability. It maintains up to 90% of the first reversible capacity after 50 cycles (521 mA h g−1 ). After 30 charge/discharge cycles, the average discharge capacities reach 472, 397 and 340 mA h g−1 even at the very high current densities of 1, 2 and 3 A g−1 , respectively. The enhanced lithium storage properties is likely to be attributed to the compact and regular crystal structure with a moderate thickness and crystallinity, which is beneficial for maintaining the stability of the structure and fast ion transport during lithiation/delithiation processes.

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Acknowledgments This work was financially supported by the One Hundred Talent Program of CAS and NSFC (21273222). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.08.154. References [1] J.T. Zai, K.X. Wang, Y.Z. Su, X.F. Qian, J.S. Chen, High stability and superior rate capability of three-dimensional hierarchical SnS2 microspheres as anode material in lithium ion batteries, J. Power Sources 196 (2011) 3650. [2] Y.P. Wu, E. Rahm, R. Holze, Carbon anode materials for lithium ion batteries, J. Power Sources 114 (2003) 228. [3] G. Zhou, D.W. Wang, L. Li, N. Li, F. Li, H.M. Cheng, Nanosize SnO2 confined in the porous shells of carbon cages for kinetically efficient and long-term lithium storage, Nanoscale 5 (2013) 1576. [4] W.Q. Zeng, F.P. Zheng, R.Z. Li, Y. Zhan, Y.Y. Li, J.P. Liu, Template synthesis of SnO2 /␣-Fe2 O3 nanotube array for 3D lithium ion battery anode with large areal capacity, Nanoscale 4 (2012) 2760. [5] L.L. Wang, T. Fei, J.A. Deng, Z. Lou, R. Wang, T. Zhang, Synthesis of rattle-type SnO2 structures with porous shell, J. Mater. Chem. 22 (2012) 18111. [6] J. Sun, C.H. Xu, L. Gao, Controllable Synthesis of Monodisperse Ultrathin SnO2 Nanorods on Nitrogen-doped Graphene and Its Ultrahigh Lithium Storage Properties, Nanoscale 4 (2012) 5425. [7] S.K. Park, S.H. Yu, N. Pinna, S. Woo, B. Jang, Y.H. Chung, Y.H. Cho, Y.E. Sung, Y. Piao, A facile hydrazine-assisted hydrothermal method for the deposition of monodisperse SnO2 nanoparticles onto graphene for lithium ion batteries, J. Mater. Chem. 22 (2012) 2520. [8] C.H. Yim, E.A. Baranova, F.M. Courtel, Y. Abu-Lebdeh, I.J. Davidson, Synthesis and characterization of macroporous tin oxide composite as an anode material for Li-ion batteries, J. Power Sources 196 (2011) 9731. [9] X.K. Wang, Z.Q. Li, Q. Li, C.B. Wang, A.L. Chen, Z.W. Zhang, R.H. Fan, L.W. Yin, Ordered mesoporous SnO2 with a highly crystalline state as an anode material for lithium ion batteries with enhanced electrochemical performance, CrystEngComm 15 (2013) 3696. [10] J.P. Liu, D.F. Xue, Sn-based nanomaterials converted from SnS nanobelts: Facile synthesis, characterizations, optical properties and energy storage performances, Electrochim. Acta 56 (2010) 243. [11] Q.H. Wang, D.W. Wang, M.H. Wu, B.X. Liu, J.T. Chen, T.M. Wang, J. Chen, Porous SnO2 nanoflakes with loose-packed structure: Morphology conserved transformation from SnS2 precursor and application in lithium ion batteries and gas sensors, J. Phys. Chem. Solids 72 (2011) 630. [12] X.M. Sun, J.F. Liu, Y.D. Li, Oxides@C Core-Shell Nanostructures: One-Pot Synthesis, Rational Conversion, and Li Storage Property, Chem. Mater. 18 (2006) 3486. [13] T. Brousse, S.M. Lee, L. Pasquereau, D. Defives, D.M. Schleich, Composite negative electrodes for lithium ion cells, Solid State Ionics 113 (1998) 51. [14] J.F. Yin, H.Q. Cao, Z.F. Zhou, J.X. Zhang, M.Z. Qu, SnS2 @reduced graphene oxide nanocomposites as anode materials with high capacity for rechargeable lithium ion batteries, J. Mater. Chem. 22 (2012) 23963. [15] J.W. Seo, J.T. Jang, S.W. Park, C.J. Kim, B.W. Park, J.W. Cheon, Two-Dimensional SnS2 Nanoplates with Extraordinary High Discharge Capacity for Lithium Ion Batteries, Adv. Mater. 20 (2008) 4269. [16] G. Domingo, R.S. Itoga, C.R. Kannewurf, Fundamental Optical Absorption in SnS2 and SnSe2 , Phys. Rev. 143 (1966) 536. [17] J. Liu, Y.C. Zhou, J.B. Wang, Y. Pan, D.F. Xue, Template-free solvothermal synthesis of yolk-shell V2 O5 microspheres as cathode materials for Li-ion batteries, Chem. Commun. 47 (2011) 10380.

447

[18] J. Liu, H. Xia, D.F. Xue, L. Lu, Double-Shelled Nanocapsules of V2 O5 -Based Composites as High-Performance Anode and Cathode Materials for Li Ion Batteries, J. Am. Chem. Soc. 131 (2009) 12086. [19] C.X. Zhai, N. Du, H. Zhang, D.R. Yang, Large-scale synthesis of ultrathin hexagonal tin disulfide nanosheets with highly reversible lithium storage, Chem. Commun. 47 (2011) 1270. [20] J.M. Ma, D.N. Lei, L. Mei, X.C. Duan, Q.H. Li, T.H. Wang, W.J. Zheng, Plate-like SnS2 nanostructures: Hydrothermal preparation, growth mechanism and excellent electrochemical properties, CrystEngComm 14 (2012) 832. [21] S.G. Patil, R.H. Tredgold, Electrical and photoconductive properties of SnS2 crystals, J. Phys. D: Appl. Phys. 4 (1971) 718. [22] J. Xia, G.C. Li, Y.C. Mao, Y.Y. Li, P.K. Shen, L.P. Chen, Hydrothermal Growth of SnS2 Hollow Spheres and Their Electrochemical Properties, CrystEngComm 14 (2012) 4279. [23] J.M. Ma, D.N. Lei, X.C. Duan, Q.H. Li, T.H. Wang, A.M. Cao, Y.H. Mao, W.J. Zheng, Designable fabrication of flower-like SnS2 aggregates with excellent performance in lithium-ion batteries, RSC Adv. 2 (2012) 3615. [24] J.H. Peng, Y.H. Xu, H. Wu, M. Hojamberdiev, Y.H. Fu, J. Wang, G.Q. Zhu, Self-assembly of SnS2 submicron-sized flakes to form microspheres under template-free hydrothermal conditions, J. Alloys Compd. 490 (2010) L20. [25] A. Chakrabarti, J. Lu, A.M. McNamara, L.M. Kuta, S.M. Stanley, Z.L. Xiao, J.A. Maguire, N.S. Hosmane, Tin(IV) sulfide: Novel nanocrystalline morphologies, Inorg. Chim. Acta 374 (2011) 627. [26] H.S. Kim, Y.H. Chung, S.H. Kang, Y.E. Sung, Electrochemical behavior of carboncoated SnS2 for use as the anode in lithium-ion batteries, Electrochim. Acta 54 (2009) 3606. [27] Y.P. Du, Z.Y. Yin, X.H. Rui, Z.Y. Zeng, X.J. Wu, J.Q. Liu, Y.Y. Zhu, J.X. Zhu, X. Huang, Q.Y. Yan, H. Zhang, Facile, relative green, inexpensive synthetic approach toward large-scale SnS2 nanoplates for high-performance lithium-ion batteries, Nanoscale 5 (2013) 1456. [28] C.X. Zhai, N. Du, H. Zhang, J.X. Yu, D.R. Yang, Multiwalled Carbon Nanotubes Anchored with SnS2 Nanosheets as High-Performance Anode Materials of Lithium-Ion Batteries, Acs Appl. Mater. Interfaces 3 (2011) 4067. [29] B. Luo, Y. Fang, B. Wang, J.S. Zhou, H.H. Song, L.J. Zhi, Two dimensional grapheneSnS2 hybrids with superior rate capability for lithium ion storage, Energy Environ. Sci. 5 (2012) 5226. [30] L.H. Zhuo, Y.Q. Wu, L.Y. Wang, Y.C. Yu, X.B. Zhang, F.Y. Zhao, One-step hydrothermal synthesis of SnS2 /graphene composites as anode material for highly efficient rechargeable lithium ion batteries, RSC Adv. 2 (2012) 5084. [31] K. Chang, Z. Wang, G.C. Huang, H. Li, W.X. Chen, J.Y. Lee, Few-layer SnS2 /graphene hybrid with exceptional electrochemical performance as lithium-ion battery anode, J. Power Sources 201 (2012) 259. [32] T.J. Kim, C. Kirn, D. Son, M. Choi, B. Park, Novel SnS2 -nanosheet anodes for lithium-ion batteries, J. Power Sources 167 (2007) 529. [33] I. Lefebvre-Devos, J. Olivier-Fourcade, J.C. Jumas, P. Lavela, Lithium insertion mechanism in SnS2 , Phys. Rev. B 61 (2000) 3110. [34] C.F. Shen, L.Y. Ma, M.B. Zheng, B. Zhao, D.F. Qiu, L.J. Pan, J.M. Cao, Y. Shi, Synthesis and electrochemical properties of graphene-SnS2 nanocomposites for lithiumion batteries, J. Solid State Electrochem. 16 (2012) 1999. [35] N. Liu, H. Wu, M.T. McDowell, Y. Yao, C. Wang, Y. Cui, A yolk-shell design for stabilized and scalable li-ion battery alloy anodes, Nano Lett. 12 (2012) 3315. [36] J. Wang, J. Liu, H.B. Xu, S.M. Ji, J.B. Wang, Y.C. Zhou, P. Hodgson, Y.C. Li, Gramscale and template-free synthesis of ultralong tin disulfide nanobelts and their lithium ion storage performances, J. Mater. Chem. A 1 (2013) 1117. [37] P. Wu, N. Du, H. Zhang, J. Liu, L. Chang, L. Wang, D. Yang, J.Z. Jiang, Layer-stacked tin disulfide nanorods in silica nanoreactors with improved lithium storage capabilities, Nanoscale 4 (2012) 4002. [38] K. Chang, W.X. Chen, H. Li, H. Li, Microwave-assisted synthesis of SnS2 /SnO2 composites by l-cysteine and their electrochemical performances when used as anode materials of Li-ion batteries, Electrochem. Acta 56 (2011) 2856. [39] S.A. Liu, X.M. Yin, Q.Y. Hao, M. Zhang, L.M. Li, L.B. Chen, Q.H. Li, Y.G. Wang, T.H. Wang, Chemical bath deposition of SnS2 nanowall arrays with improved electrochemical performance for lithium ion battery, Mater. Lett. 64 (2010) 2350.