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Hierarchical tin-based microspheres: Solvothermal synthesis, chemical conversion, mechanism and application in lithium ion batteries
Electrochimica Acta 106 (2013) 386–391
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hierarchical tin-based microspheres: Solvothermal synthesis, chemical conversion, mechanism and application in lithium ion batteries Danni Lei, Ming Zhang, Baihua Qu, Jianmin Ma, Qiuhong Li, Libao Chen, Bingan Lu, Taihong Wang ∗ Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, and State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, China
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 26 May 2013 Accepted 27 May 2013 Available online 2 June 2013 Keywords: SnS2 SnO2 Microsphere Lithium ion battery
a b s t r a c t Uniform hierarchical SnS2 structures were synthesized by a facile solvothermal approach. Based on timedependent experiments results, a possible formation mechanism of the hierarchical SnS2 architectures was proposed. Mesoporous hierarchical SnO2 also could be obtained by calcining the corresponding SnS2 structures. When used as the anode materials of rechargeable lithium-ion batteries, both of them showed high specific capacities and enhanced rate capacities. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction As one kind of energy storage devices, lithium-ion batteries (LIBs) have been extremely important power sources for various portable electronic devices and electric vehicles in modern society [1–5]. To keep pace with the rapid development of integrated circuits and their high energy requirements, new materials with better performance are urgently needed. Among various anode materials, tin-based materials have been extensively studied as possible alternatives for commercially available carbon electrodes due to their high theoretical capacity for battery application (781 mAh g−1 for SnO2 ; 645 mAh g−1 for SnS2 ), low cost, low toxicity, and widespread availability [6–13]. Recently, inorganic hierarchical nanostructures with hollow interiors have attracted increasing attention in many fields, because of their extraordinarily high activated surface and roust stability [14–18]. SnS2 materials have shown good cycling stability as anode for LIBs because of their two-dimensional (2D)-layered structures [19–21]. However, electrochemical performances of such materials still need to be improved. Consequently, the morphology controls of SnS2 nanomaterials are actively pursued. In particular, much efforts have been devoted to rational and skillful control of
∗ Corresponding author. E-mail address: [email protected] (T. Wang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.099
hierarchical and complex nanostructures self-assembled with nanosheets [22,23]. Recently, Zai et al. synthesized threedimensional (3D) architectures with a red-embroidery-ball-like structure, which significantly enhance their lithium storage capacity and improve rate performance [24]. The improvement for such 3D hierarchical structures are related to their structural characteristics, such as large surface area, greater accessibility to electrolyte, faster transportation of Li+ , and accelerated phase transitions. However, there are only a few reports on the synthesis of 3D SnS2 hierarchical structures. Thus, we have developed an alternative method to prepare SnS2 with 3D hierarchical structure. On the other hand, SnO2 materials with hierarchical structure can take both the advantages of the nanometer size effects and the high stability arisen from the micro- or sub-micro-sized assemblies [25–27]. The conventional fabrication procedures of hollow structure generally involved hard templates or soft templates [28–30], which related to tedious synthesis and high cost. Thus, it is of great significance to develop a facile method to synthesize SnO2 hierarchical nanostructures with hollow interiors from the view of both scientific research and practical application. In this work, a facile solvothermal approach was developed to fabricate hierarchical SnS2 microspheres. The formation mechanism of the hierarchical SnS2 microspheres was proposed based on the systematic study. In addition, mesoporous hierarchical SnO2 microspheres have been obtained through calcining the corresponding SnS2 precursors while preserving their original
D. Lei et al. / Electrochimica Acta 106 (2013) 386–391
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Fig. 2. TGA and DSC curves of the SnS2 sample.
Fig. 1. The typical XRD patterns of the hierarchical SnS2 microspheres and hierarchical SnO2 microspheres.
morphologies. When they were used as the anode for LIBs, these unique hierarchical tin-based microspheres can significantly enhance their lithium storage capacity, respectively. 2. Experimental 2.1. Preparation of hierarchical SnS2 and SnO2 microspheres All the chemicals were of analytical grade and used as received without further purification. In a typical procedure, 1.4 g tetrachlorostannane pentahydrate (SnCl4 ·5H2 O), 1.2 g thioacetamide (TAA) was dissolved in 100 mL ethanol to obtain the homogenous solution. After being stirred for several minutes, the clear solution was transferred into a 130 mL Teflon-lined autoclave and maintained at 180 ◦ C for 24 h, then cooled to room temperature naturally. The precipitate was collected by centrifugation after being washed with distilled water and ethanol several times. Finally, the product was dried completely in vacuum at 80 ◦ C. The
mesoporous SnO2 superstructures could be obtained by annealing the as-prepared SnS2 in a muffle furnace at 500 ◦ C in air with heating rate of 2 ◦ C min−1 . 2.2. Characterization The crystal structure of the products were determined by ˚ X-ray powder diffraction (XRD, Cu K␣ radiation; = 1.5408 A) with a SIEMENS D5000 X-ray diffractometer. Scanning electron microscope (SEM) images were performed with a Hitachi S-4800 microscope. Thermogravimetry analysis was carried out on a Netzsch STA449C (at a heating rate of 10 ◦ C min−1 in flow air). 2.3. Electrochemical measurement The electrochemical properties of products were measured using CR 2025-type coin cells. In a process of fabricating the LIBs, The anode electrode consisted of 80 wt% active material, 10 wt% conductive carbon black, and 10 wt% binder (carboxyl methyl cellulose) on a copper foil. One molar LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl
Fig. 3. (a–c) SEM images of hierarchical SnS2 microspheres; (d–f) SEM images of hierarchical SnO2 microspheres.
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Fig. 4. (a, b) TEM images of hierarchical SnS2 microspheres; (c, d) TEM images of hierarchical SnO2 microspheres.
carbonate (EMC) (EC:DMC:EMC = 1:1:1) was used as the electrolyte. The assembly of the test cells was performed in an argon-filled glove box with water and oxygen contents less than 1 ppm using pure lithium foils as the counter electrode, and a polypropylene (PP) film (Celgard 2400) as the separator. The cells were then aged
for 8 h before measurement. The electrochemical measurements were carried out with a multi-channel current static system (Arbin Instruments BT 2000, USA). The electrochemical performances of the cells were evaluated within the potential range of 0.05–2 V versus Li/Li+ (1 M).
Fig. 5. SEM images of obtained SnS2 sample at 180 ◦ C at different time: 20 min (a), 40 min (b), 1 h (c), and 12 h (d).
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3. Results and discussion 3.1. Structure and morphology characterization To confirm crystal structures of the sample, XRD characterizations were performed. Fig. 1 shows the typical XRD patterns of the samples before annealed. The unannealed products display the characteristic peaks corresponding to SnS2 (JCPDS no. 23-667). No other peaks are observed, indicating the high purity of SnS2 precursors synthesized at 180 ◦ C via the solvothermal route. Thermogravimetric (TG–DSC) curve of the precursor SnS2 is displayed in Fig. 2. It is found that the SnS2 can stabilize to a temperature as high as 400 ◦ C. Above 400 ◦ C, there is a well-defined weight loss of about 15.1 wt%, which is due to the decomposition of SnS2 to SnO2 . This result matches well with the theoretical value (17.5 wt%) [31] for the substitution of S by O atoms from SnS2 to SnO2 , which further support the oxidizing process of SnS2 precursors. Below 400 ◦ C, there is only a little mass loss, which can be related to the evaporation of adsorbed water and/or water trap within the interlayers [31]. According to the TG–DSC result, we annealed the SnS2 at 500 ◦ C for 2 h in air aiming to obtain the high purity of SnO2 . All the diffraction peaks in the XRD pattern (Fig. 1) of the sample via calcination can be readily indexed to the SnO2 standard card (JCPDS no.41-1445) and no XRD peaks arising from SnS2 . Therefore, SnO2 was achieved by calcining SnS2 at 500 ◦ C. Visible morphologies and structures of both SnS2 and SnO2 samples were further illuminated by scanning electron microcopy (SEM), as shown in Fig. 3. Panoramic SEM image (Fig. 3a) confirms the large-scale and uniform production of the SnS2 microspheres. The detailed morphology of the SnS2 microspheres in Fig. 3b reveals that the entire structure of the architecture is built of several dozen nanoplates with smooth surface. Such nanoplates connect to each other through the center to form porous 3D flowerlike structures. Fig. 3c reveals that the thickness of nanoplates is approximately 10 nm. From the low-magnification SEM image (Fig. 3d) of the as-prepared SnO2 product, it can be seen that the as-obtained sample is composed of nanoplates. These nanoplates interweave with each other in a random fashion to reveal a three-dimensional characteristic appearance and the formation of loosely packed microstructure in the nanometer scale. Interestingly, as shown in the high-magnification SEM images (Fig. 3e and f), there are numerous pores on the surface of the nanoplates, indicating that porous structures are formed after the annealing process. In fact, such a pores feature is the benefit of Li-ion diffusion [32]. TEM studies were also carried out in order to give the inherent structure information of the SnS2 and SnO2 microspheres. Fig. 4a represents a high magnification TEM image of an individual petal of the SnS2 microspheres. These nanosheets are long layer-rolled and undulating. Fig. 4b is the magnified cross-sectional view of the SnS2 nanosheet. It exhibits parallel fringes with a spacing of 0.279 nm, which can be assigned to the (1 0 1) plane of a hexagonal phase SnS2 (JCPDS no. 23-667). From the high magnification TEM image of SnO2 (Fig. 4c), abundant porous structures are formed among the nanoparticles, which are consistent with the FESEM result. A typical HRTEM image (Fig. 4d) shows the clear lattice fringes indicating that the SnO2 was well crystallized. Lattice plane spacings calculated from the HRTEM image are 0.264 nm and 0.331 nm that correspond to (1 0 1) and (1 1 0) crystal plane of rutile type SnO2 (JCPDS no. 41-1445), respectively. 3.2. Growth mechanism of the hierarchical SnS2 microspheres In our reaction system, the formation mechanism of the hydrothermal reaction is proposed based on the SEM images observed for different reaction times, as shown in Fig. 5. Based on these SEM images, the formation of the hierarchical SnS2
Fig. 6. (a) Discharge–charge voltage profiles at the 0.1 C rate for the first cycle, (b) cycling performance at the 0.1 C rate, and (c) the rate capability of SnS2 and SnO2 electrodes.
microspheres could be described as following steps: (i) SnS2 precursors precipitated to become the nuclei and quickly grew into the primary particles, then aggregated in microparticles, as shown in Fig. 5a; (ii) with the reaction time to 40 min (Fig. 5b), the interconnected microparticles further grew and self-organized into disperse flowerlike structure to minimize the surface energy; (iii) under further hydrothermal reaction time to 1 h (Fig. 5c), the amount of dispersive flowers increased at the expense of microparticles and the size of the nanosheets grew gradually; (iv) as shown in Fig. 5d, no microparticles remained and sample was composed entirely of the 3D hierarchical microspheres. The above process was obviously controlled by the Ostwald Ripening and crystal growth habit.
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3.3. Electrochemical characteristics In this research, the specific capacity and cycling stability of electrodes were measured by constant current charge/discharge testing. Fig. 6a shows the voltage profiles of SnS2 and SnO2 electrochemical cells at a constant current density of 0.1 C (1 C = 800 mA g−1 ) between 0.05 and 2.0 V (vs. Li/Li+ ). The cyclic properties and the discharge capacity for SnS2 and SnO2 electrodes are shown in Fig. 6b. As compared with SnS2 electrodes, the SnO2 has larger capacity because of its higher theoretical capacity. The initial capacity of SnO2 is 1645 mAh g−1 , and decreases gradually to 566 mAh g−1 after 50 cycles, while the capacity of SnS2 decreases from 1500 to 475 mAh g−1 . The above results demonstrate that the cycling performances for the two nanostructures are closely related to the different micro-structures. Fig. 6c shows rate performances of SnS2 and SnO2 electrodes at different charge current densities between 0.05 and 2.0 V while the discharge current densities were held constant (0.1 C). It can be seen that SnS2 and SnO2 electrodes exhibit a stable capacity and good rate performances. For the SnS2 electrodes, a specific charge capacity of 749 mAh g−1 is obtained at a rate of 0.1 C after 5 cycles; the average charge capacity of 700, 640, 600, 500, 410 mAh g−1 is observed at 0.2 C, 0.5 C, 1 C, 2 C, 5 C, respectively. For comparison, the reversible charge capacity of the SnO2 electrodes at the rate of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C are 880, 800, 720, 680, 600, 490 mAh g−1 , separately. When the charge rate was brought back to 0.1 C after 5 cycles at 5 C, the charge capacity of SnO2 electrode could recover to 690 mAh g−1 . On the contrary, the SnS2 electrodes show lower capacity. Their excellent lithium storage properties could be explained as follows: (i) the porous structure allows the electrolyte to easily diffuse into the interior of the composite particle and reduce the resistance of transported Li ion; (ii) the high surface area increases the contact between the electrode/electrolyte and provides a large number of potential active sites for the Li+ transfer reaction. The high capacity achieved at a high cycling rate implies that this type of electrode can be a promising candidate for high power applications. 4. Conclusion In summary, uniform hierarchical SnS2 microspheres were synthesized by a facile solvothermal approach. A possible formation mechanism of the hierarchical SnS2 architectures was proposed. In addition, mesoporous SnO2 superstructures have been successfully obtained by calcining the corresponding SnS2 precursors while preserving their original morphologies. As the anode materials for rechargeable Li-ion batteries, these unique structures can significantly enhance their lithium storage capacity. These hierarchical structure assembled by interconnected nanoplates could be a promising candidate for high power LIBs. Acknowledgments This work was partly supported from Hunan Provincial Natural Science Foundation of China (Grant Nos. 10JJ1011, 11FH003), National Natural Science Foundation of China (Grant Nos. 21003041, 21103046) and the Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2012B125). References [1] K. Sato, M. Noguchi, A. Demachi, N. Oki, M. Endo, A mechanism of lithium storage in disordered carbons, Science 264 (1994) 556. [2] M.S. Whittingham, Lithium batteries and cathode materials, Chemical Reviews 104 (2004) 4271. [3] F.Y. Cheng, Z.L. Tao, J. Liang, J. Chen, Template-directed materials for rechargeable lithium-ion batteries, Chemistry of Materials 20 (2008) 667.
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Wang, Synthesis of self-assembled 3D flowerlike SnS2 nanostructures with enhanced lithium ion storage property, Solid State Sciences 12 (2010) 712. [23] J.T. Zai, X.F. Qian, K.X. Wang, C. Yu, L.Q. Tao, Y.L. Xiao, J.S. Chen, 3Dhierarchical SnS2 micro/nano-structures: controlled synthesis, formation mechanism and lithium ion storage performances, CrystEngCommun 14 (2012) 1364. [24] J.T. Zai, K.X. Wang, Y.Z. Sub, 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, Journal of Power Sources 196 (2011) 3650. [25] A.M. Cao, J.S. Hu, H.P. Liang, L.J. Wan, Self-assembled vanadium pentoxide (V2 O5 ) hollow microspheres from nanorods and their application in lithium-ion batteries, Angewandte Chemie International Edition 44 (2005) 4391. [26] S.Y. Gao, S.X. Yang, J. Shu, S.X. Zhang, Z.D. Li, K. Jiang, Green fabrication of hierarchical CuO hollow micro/nanostructures and enhanced performance as electrode materials for lithium-ion batteries, Journal of Physical Chemistry C 112 (2008) 19324. [27] X.M. Yin, C.C. Li, M. Zhang, Q.Y. Hao, S. Liu, L.B. Chen, T.H. Wang, One-step synthesis of hierarchical SnO2 hollow nanostructures via self-assembly for high power lithium ion batteries, Journal of Physical Chemistry C 114 (2010) 8084. [28] Y.D. Xia, R. Mokaya, Hollow spheres of crystalline porous metal oxides: a generalized synthesis route via nanocasting with mesoporous carbon hollow shells, Journal of Materials Chemistry 15 (2005) 3126. [29] H.L. Xu, W.Z. Wang, Template Synthesis of multishelled Cu2 O hollow spheres with a single-crystalline shell wall, Angewandte Chemie International Edition 46 (2007) 1489.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hierarchical tin-based microspheres: Solvothermal synthesis, chemical conversion, mechanism and application in lithium ion batteries Danni Lei, Ming Zhang, Baihua Qu, Jianmin Ma, Qiuhong Li, Libao Chen, Bingan Lu, Taihong Wang ∗ Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, and State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, China
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 26 May 2013 Accepted 27 May 2013 Available online 2 June 2013 Keywords: SnS2 SnO2 Microsphere Lithium ion battery
a b s t r a c t Uniform hierarchical SnS2 structures were synthesized by a facile solvothermal approach. Based on timedependent experiments results, a possible formation mechanism of the hierarchical SnS2 architectures was proposed. Mesoporous hierarchical SnO2 also could be obtained by calcining the corresponding SnS2 structures. When used as the anode materials of rechargeable lithium-ion batteries, both of them showed high specific capacities and enhanced rate capacities. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction As one kind of energy storage devices, lithium-ion batteries (LIBs) have been extremely important power sources for various portable electronic devices and electric vehicles in modern society [1–5]. To keep pace with the rapid development of integrated circuits and their high energy requirements, new materials with better performance are urgently needed. Among various anode materials, tin-based materials have been extensively studied as possible alternatives for commercially available carbon electrodes due to their high theoretical capacity for battery application (781 mAh g−1 for SnO2 ; 645 mAh g−1 for SnS2 ), low cost, low toxicity, and widespread availability [6–13]. Recently, inorganic hierarchical nanostructures with hollow interiors have attracted increasing attention in many fields, because of their extraordinarily high activated surface and roust stability [14–18]. SnS2 materials have shown good cycling stability as anode for LIBs because of their two-dimensional (2D)-layered structures [19–21]. However, electrochemical performances of such materials still need to be improved. Consequently, the morphology controls of SnS2 nanomaterials are actively pursued. In particular, much efforts have been devoted to rational and skillful control of
∗ Corresponding author. E-mail address: [email protected] (T. Wang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.099
hierarchical and complex nanostructures self-assembled with nanosheets [22,23]. Recently, Zai et al. synthesized threedimensional (3D) architectures with a red-embroidery-ball-like structure, which significantly enhance their lithium storage capacity and improve rate performance [24]. The improvement for such 3D hierarchical structures are related to their structural characteristics, such as large surface area, greater accessibility to electrolyte, faster transportation of Li+ , and accelerated phase transitions. However, there are only a few reports on the synthesis of 3D SnS2 hierarchical structures. Thus, we have developed an alternative method to prepare SnS2 with 3D hierarchical structure. On the other hand, SnO2 materials with hierarchical structure can take both the advantages of the nanometer size effects and the high stability arisen from the micro- or sub-micro-sized assemblies [25–27]. The conventional fabrication procedures of hollow structure generally involved hard templates or soft templates [28–30], which related to tedious synthesis and high cost. Thus, it is of great significance to develop a facile method to synthesize SnO2 hierarchical nanostructures with hollow interiors from the view of both scientific research and practical application. In this work, a facile solvothermal approach was developed to fabricate hierarchical SnS2 microspheres. The formation mechanism of the hierarchical SnS2 microspheres was proposed based on the systematic study. In addition, mesoporous hierarchical SnO2 microspheres have been obtained through calcining the corresponding SnS2 precursors while preserving their original
D. Lei et al. / Electrochimica Acta 106 (2013) 386–391
387
Fig. 2. TGA and DSC curves of the SnS2 sample.
Fig. 1. The typical XRD patterns of the hierarchical SnS2 microspheres and hierarchical SnO2 microspheres.
morphologies. When they were used as the anode for LIBs, these unique hierarchical tin-based microspheres can significantly enhance their lithium storage capacity, respectively. 2. Experimental 2.1. Preparation of hierarchical SnS2 and SnO2 microspheres All the chemicals were of analytical grade and used as received without further purification. In a typical procedure, 1.4 g tetrachlorostannane pentahydrate (SnCl4 ·5H2 O), 1.2 g thioacetamide (TAA) was dissolved in 100 mL ethanol to obtain the homogenous solution. After being stirred for several minutes, the clear solution was transferred into a 130 mL Teflon-lined autoclave and maintained at 180 ◦ C for 24 h, then cooled to room temperature naturally. The precipitate was collected by centrifugation after being washed with distilled water and ethanol several times. Finally, the product was dried completely in vacuum at 80 ◦ C. The
mesoporous SnO2 superstructures could be obtained by annealing the as-prepared SnS2 in a muffle furnace at 500 ◦ C in air with heating rate of 2 ◦ C min−1 . 2.2. Characterization The crystal structure of the products were determined by ˚ X-ray powder diffraction (XRD, Cu K␣ radiation; = 1.5408 A) with a SIEMENS D5000 X-ray diffractometer. Scanning electron microscope (SEM) images were performed with a Hitachi S-4800 microscope. Thermogravimetry analysis was carried out on a Netzsch STA449C (at a heating rate of 10 ◦ C min−1 in flow air). 2.3. Electrochemical measurement The electrochemical properties of products were measured using CR 2025-type coin cells. In a process of fabricating the LIBs, The anode electrode consisted of 80 wt% active material, 10 wt% conductive carbon black, and 10 wt% binder (carboxyl methyl cellulose) on a copper foil. One molar LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl
Fig. 3. (a–c) SEM images of hierarchical SnS2 microspheres; (d–f) SEM images of hierarchical SnO2 microspheres.
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Fig. 4. (a, b) TEM images of hierarchical SnS2 microspheres; (c, d) TEM images of hierarchical SnO2 microspheres.
carbonate (EMC) (EC:DMC:EMC = 1:1:1) was used as the electrolyte. The assembly of the test cells was performed in an argon-filled glove box with water and oxygen contents less than 1 ppm using pure lithium foils as the counter electrode, and a polypropylene (PP) film (Celgard 2400) as the separator. The cells were then aged
for 8 h before measurement. The electrochemical measurements were carried out with a multi-channel current static system (Arbin Instruments BT 2000, USA). The electrochemical performances of the cells were evaluated within the potential range of 0.05–2 V versus Li/Li+ (1 M).
Fig. 5. SEM images of obtained SnS2 sample at 180 ◦ C at different time: 20 min (a), 40 min (b), 1 h (c), and 12 h (d).
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3. Results and discussion 3.1. Structure and morphology characterization To confirm crystal structures of the sample, XRD characterizations were performed. Fig. 1 shows the typical XRD patterns of the samples before annealed. The unannealed products display the characteristic peaks corresponding to SnS2 (JCPDS no. 23-667). No other peaks are observed, indicating the high purity of SnS2 precursors synthesized at 180 ◦ C via the solvothermal route. Thermogravimetric (TG–DSC) curve of the precursor SnS2 is displayed in Fig. 2. It is found that the SnS2 can stabilize to a temperature as high as 400 ◦ C. Above 400 ◦ C, there is a well-defined weight loss of about 15.1 wt%, which is due to the decomposition of SnS2 to SnO2 . This result matches well with the theoretical value (17.5 wt%) [31] for the substitution of S by O atoms from SnS2 to SnO2 , which further support the oxidizing process of SnS2 precursors. Below 400 ◦ C, there is only a little mass loss, which can be related to the evaporation of adsorbed water and/or water trap within the interlayers [31]. According to the TG–DSC result, we annealed the SnS2 at 500 ◦ C for 2 h in air aiming to obtain the high purity of SnO2 . All the diffraction peaks in the XRD pattern (Fig. 1) of the sample via calcination can be readily indexed to the SnO2 standard card (JCPDS no.41-1445) and no XRD peaks arising from SnS2 . Therefore, SnO2 was achieved by calcining SnS2 at 500 ◦ C. Visible morphologies and structures of both SnS2 and SnO2 samples were further illuminated by scanning electron microcopy (SEM), as shown in Fig. 3. Panoramic SEM image (Fig. 3a) confirms the large-scale and uniform production of the SnS2 microspheres. The detailed morphology of the SnS2 microspheres in Fig. 3b reveals that the entire structure of the architecture is built of several dozen nanoplates with smooth surface. Such nanoplates connect to each other through the center to form porous 3D flowerlike structures. Fig. 3c reveals that the thickness of nanoplates is approximately 10 nm. From the low-magnification SEM image (Fig. 3d) of the as-prepared SnO2 product, it can be seen that the as-obtained sample is composed of nanoplates. These nanoplates interweave with each other in a random fashion to reveal a three-dimensional characteristic appearance and the formation of loosely packed microstructure in the nanometer scale. Interestingly, as shown in the high-magnification SEM images (Fig. 3e and f), there are numerous pores on the surface of the nanoplates, indicating that porous structures are formed after the annealing process. In fact, such a pores feature is the benefit of Li-ion diffusion [32]. TEM studies were also carried out in order to give the inherent structure information of the SnS2 and SnO2 microspheres. Fig. 4a represents a high magnification TEM image of an individual petal of the SnS2 microspheres. These nanosheets are long layer-rolled and undulating. Fig. 4b is the magnified cross-sectional view of the SnS2 nanosheet. It exhibits parallel fringes with a spacing of 0.279 nm, which can be assigned to the (1 0 1) plane of a hexagonal phase SnS2 (JCPDS no. 23-667). From the high magnification TEM image of SnO2 (Fig. 4c), abundant porous structures are formed among the nanoparticles, which are consistent with the FESEM result. A typical HRTEM image (Fig. 4d) shows the clear lattice fringes indicating that the SnO2 was well crystallized. Lattice plane spacings calculated from the HRTEM image are 0.264 nm and 0.331 nm that correspond to (1 0 1) and (1 1 0) crystal plane of rutile type SnO2 (JCPDS no. 41-1445), respectively. 3.2. Growth mechanism of the hierarchical SnS2 microspheres In our reaction system, the formation mechanism of the hydrothermal reaction is proposed based on the SEM images observed for different reaction times, as shown in Fig. 5. Based on these SEM images, the formation of the hierarchical SnS2
Fig. 6. (a) Discharge–charge voltage profiles at the 0.1 C rate for the first cycle, (b) cycling performance at the 0.1 C rate, and (c) the rate capability of SnS2 and SnO2 electrodes.
microspheres could be described as following steps: (i) SnS2 precursors precipitated to become the nuclei and quickly grew into the primary particles, then aggregated in microparticles, as shown in Fig. 5a; (ii) with the reaction time to 40 min (Fig. 5b), the interconnected microparticles further grew and self-organized into disperse flowerlike structure to minimize the surface energy; (iii) under further hydrothermal reaction time to 1 h (Fig. 5c), the amount of dispersive flowers increased at the expense of microparticles and the size of the nanosheets grew gradually; (iv) as shown in Fig. 5d, no microparticles remained and sample was composed entirely of the 3D hierarchical microspheres. The above process was obviously controlled by the Ostwald Ripening and crystal growth habit.
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3.3. Electrochemical characteristics In this research, the specific capacity and cycling stability of electrodes were measured by constant current charge/discharge testing. Fig. 6a shows the voltage profiles of SnS2 and SnO2 electrochemical cells at a constant current density of 0.1 C (1 C = 800 mA g−1 ) between 0.05 and 2.0 V (vs. Li/Li+ ). The cyclic properties and the discharge capacity for SnS2 and SnO2 electrodes are shown in Fig. 6b. As compared with SnS2 electrodes, the SnO2 has larger capacity because of its higher theoretical capacity. The initial capacity of SnO2 is 1645 mAh g−1 , and decreases gradually to 566 mAh g−1 after 50 cycles, while the capacity of SnS2 decreases from 1500 to 475 mAh g−1 . The above results demonstrate that the cycling performances for the two nanostructures are closely related to the different micro-structures. Fig. 6c shows rate performances of SnS2 and SnO2 electrodes at different charge current densities between 0.05 and 2.0 V while the discharge current densities were held constant (0.1 C). It can be seen that SnS2 and SnO2 electrodes exhibit a stable capacity and good rate performances. For the SnS2 electrodes, a specific charge capacity of 749 mAh g−1 is obtained at a rate of 0.1 C after 5 cycles; the average charge capacity of 700, 640, 600, 500, 410 mAh g−1 is observed at 0.2 C, 0.5 C, 1 C, 2 C, 5 C, respectively. For comparison, the reversible charge capacity of the SnO2 electrodes at the rate of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C are 880, 800, 720, 680, 600, 490 mAh g−1 , separately. When the charge rate was brought back to 0.1 C after 5 cycles at 5 C, the charge capacity of SnO2 electrode could recover to 690 mAh g−1 . On the contrary, the SnS2 electrodes show lower capacity. Their excellent lithium storage properties could be explained as follows: (i) the porous structure allows the electrolyte to easily diffuse into the interior of the composite particle and reduce the resistance of transported Li ion; (ii) the high surface area increases the contact between the electrode/electrolyte and provides a large number of potential active sites for the Li+ transfer reaction. The high capacity achieved at a high cycling rate implies that this type of electrode can be a promising candidate for high power applications. 4. Conclusion In summary, uniform hierarchical SnS2 microspheres were synthesized by a facile solvothermal approach. A possible formation mechanism of the hierarchical SnS2 architectures was proposed. In addition, mesoporous SnO2 superstructures have been successfully obtained by calcining the corresponding SnS2 precursors while preserving their original morphologies. As the anode materials for rechargeable Li-ion batteries, these unique structures can significantly enhance their lithium storage capacity. These hierarchical structure assembled by interconnected nanoplates could be a promising candidate for high power LIBs. Acknowledgments This work was partly supported from Hunan Provincial Natural Science Foundation of China (Grant Nos. 10JJ1011, 11FH003), National Natural Science Foundation of China (Grant Nos. 21003041, 21103046) and the Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2012B125). References [1] K. Sato, M. Noguchi, A. Demachi, N. Oki, M. Endo, A mechanism of lithium storage in disordered carbons, Science 264 (1994) 556. [2] M.S. Whittingham, Lithium batteries and cathode materials, Chemical Reviews 104 (2004) 4271. [3] F.Y. Cheng, Z.L. Tao, J. Liang, J. Chen, Template-directed materials for rechargeable lithium-ion batteries, Chemistry of Materials 20 (2008) 667.
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