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Impressive lithium storage of SnO2@TiO2 nanospheres with a yolk-like core derived from self-assembled SnO2 nanoparticles
Journal of Alloys and Compounds 702 (2017) 99e105
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Impressive lithium storage of SnO2@TiO2 nanospheres with a yolk-like core derived from self-assembled SnO2 nanoparticles Qinghua Tian a, b, *, Yang Tian b, Wei Zhang a, Jun Huang b, Zhengxi Zhang b, Li Yang b, ** a b
Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2016 Received in revised form 18 January 2017 Accepted 22 January 2017 Available online 25 January 2017
A unique structure of yolk-like SnO2@TiO2 nanospheres has been fabricated via SnO2 nanoparticles selfassembled into a core during calcination process. Moreover, this as-prepared composite owns a mesoporous structure, which can accelerate the diffusion of both electrons and lithium-ions by providing a high electrode-electrolyte contact area. When evaluated as anode material for lithium-ion batteries, the SnO2@TiO2 gives a high reversible capacity of 472.7 mAh g1 even at 2 A g1 after 800 cycles, exhibiting significantly improved electrochemical performance. This work may provide a broader vision into fabricating yolk-like SnO2@TiO2 heterostructures for high-performance anode materials of lithium-ion batteries. © 2017 Elsevier B.V. All rights reserved.
Keywords: SnO2@TiO2 Nanostructures Electrochemical properties Anode Lithium-ion batteries
1. Introduction Rechargeable lithium-ion batteries (LIBs) have attracted the most widely attention for portable electronic devices and electrical/ hybrid vehicles, due to their high operating voltage, high energy density and low self-discharge rate [1e5]. Commercial graphite anodes are commonly used as the anode materials in LIBs nowadays since their high reversibility. But, their limited theoretical capacity (about 372 mAh g1) cannot meet the pressing demands of next generation LIBs [6,7]. To meet the increasing demand for LIBs with excellent rate capability and high reversible capacity, massive efforts have been undertaken to develop new electrode materials or design unique structures of electrode materials [8e11]. As one kind of the most promising candidates for anode materials, transition metal oxides have been widely investigated for LIBs anodes to replace the commercial graphite anode due to their high theoretical capacities [12]. Among them, SnO2 has been extensively studied for lithium storage due to its abundance, low onset potential and high theoretical capacity (782 mAh g1) [13,14]. However, the practical implementation of SnO2 typically undergoes severe pulverization
* Corresponding author. Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China. ** Corresponding author. E-mail addresses: [email protected] (Q. Tian), [email protected] (L. Yang). http://dx.doi.org/10.1016/j.jallcom.2017.01.253 0925-8388/© 2017 Elsevier B.V. All rights reserved.
induced by large volume expansion (up to 250%) and agglomeration during the charge-discharge process, leading to the rapid capacity fade [15]. One of the effective strategies for improving the performance of SnO2 electrodes is fabricating heterostructures of SnO2 with other stable materials [16e18]. Considering the merit of negligible volume change (<4%) upon Liþ intercalation and de-intercalation, TiO2 has been proposed as a prospective candidate for the mechanical support of SnO2 [19e25]. There is a typical strategy has been employed, namely through designing a hollow structure of SnO2/TiO2 composite with a hollow space and one-dimensional TiO2 surface coating to stabilize the SEI and accommodate the free expansion of SnO2 anode materials, such as the core-shell nanowires or double-shell nanotubes [26e29]. Although exhibited better cycling and improved rate property compared to single component, the composites with the hollow space and one-dimensional structures have been involved in the complex synthesis processes, such as the chemical vapor deposition, electrospinning processes and even the use of high concentration of alkali etching (NaOH) [26,28,29]. Thus, it is worthy of searching facile approaches to prepare heterostructures of SnO2 with TiO2. Herein, a nanostructure of yolk-like SnO2@TiO2 has been successfully fabricated by a novel and facile method, as illustrated in Scheme 1, which effectively avoids the use of HF or high concentration of alkali etching. In the as-prepared yolk-like SnO2@TiO2
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Scheme 1. Synthesis procedure of the yolk-like SnO2@TiO2 nanospheres.
composite, the TiO2 not only works as a mechanical support which can effectively restrain the pulverization of SnO2 upon Liþ intercalation and de-intercalation, but also prevents the aggregation of the SnO2 and contributes to the capacity within a certain voltage range [30]. Moreover, the inside rich void space can also accommodate the extreme volume expansion of SnO2 during chargedischarge process [31,32]. Also the mesoporous structure of the special composite makes great contribution to improve the performance by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electrons and lithium-ions [33,34]. When evaluated as the anode material for LIBs, the yolklike SnO2@TiO2 composite with a void space exhibits a high reversible capacity of 472.7 mAh g1 at a high current density of 2000 mA g1 even after 800 cycles. 2. Experimental 2.1. Preparation of SnO2@TiO2 The polysaccharide nanospheres were first prepared by hydrothermal method according to our previous work [35]. Then, polysaccharide@SnO2 nanospheres were fabricated by depositing the SnO2 on the surface of the polysaccharide. Briefly, 100 mg polysaccharide was dispersed in the mixed solution (100 ml DI water and 100 ml ethanol) by ultrasound for 20 min. Then 120 mg SnCl2$2H2O was successively added into the mixed solution under vigorous stirring. After stirring for 24 h, the product was centrifuged and dried at 60 C for 12 h. According to the work of Wang [36], the polysaccharide@SnO2@TiO2 was prepared. Typically, the polysaccharide@SnO2 nanospheres were dispersed in 20 ml of ethanol/ acetonitrile mixture (3:1 v/v) and 0.15 ml ammonia solution (32%) was then added under vigorous stirring at room temperature. After that, 0.3 ml of titanium isopropoxide (TTIP) in 5 ml ethanol/ acetonitrile (3:1 v/v) was slowly introduced into the above suspension with stirring. After stirring for 2 h, the product was recovered by centrifugation, followed by washed with DI water for two times and dried under 60 Cfor 12 h. Finally, the SnO2@TiO2 particles were obtained by as-collected product calcination at 550 Cfor 5 h in air. It is worth noting that SnO2 nanoparticles deposited on surface of polysaccharide self-assembles into a core to reduce the whole surface energy during calcination process, finally resulting in formation of yolk-like SnO2@TiO2 nanospheres. For comparison, the SnO2 nanoparticles were also prepared by polysaccharide@SnO2 composite calcination at 550 C in air for 5 h. 2.2. Characterizations The morphologies and microstructures of the products were studied using field emitting scanning electron microscopy (FE-SEM, JEOL JSM-7401F) and transmission electron microscopy (TEM, JEOL JEM-2010) with an energy dispersive X-ray spectrometer (EDX). The crystal structure and composition were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-Rbusing Cu Ka radiation) and Inductively Coupled Plasma Mass Spectrometer (ICP-
MS, Agilent 7500a). X-Ray photoelectron spectroscopy (XPS) experiments were carried out on an AXIS UltraDLD instrument. The surface area and pore volume of the prepared materials were characterized by Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) methods based on the Surface Area and Porosimetry Analyzer (ASAP 2010 MþC). Electrochemical measurements were performed using 2016type coin cells assembled in an argon-filled glove box (German, M. Braun Co., [O2]< 1 ppm, [H2O] < 1 ppm). For preparing working electrodes, a mixture of the active material, acetylene black, and polyvinylidene fluoride (PVDF) binder at a weight ratio of 80: 10: 10 was pasted on pure copper foil. Pure lithium foil was used as the counter electrode. A glass fiber (GF/A) from Whatman was used as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (EC þDMC) (1: 1 in volume). The cells were cycled under 800 and 2000 mA g1 between cutoff voltages of 3.0 and 0.01 V on a CT2001A cell test instrument (LAND Electronic Co.) at room temperature. Cyclic voltammetry (CV) was implemented on a CHI660D electrochemical workstation. Electrochemical impedance spectrum (EIS) measurements were performed using a CHI660D electrochemical workstation in the frequency range from 100 KHz to 0.01 Hz with an ac perturbation of 5.0 mV s1.
3. Results and discussion The morphology and structure of the yolk-like SnO2@TiO2 composite were examined by the FESEM and TEM/EDX. As shown in Fig. 1a, the diameter of polysaccharide is 200e300 nm, and with a smooth surface. The morphology of polysaccharide@SnO2@TiO2 is exhibited in Fig. 1b. The diameter of polysaccharide@SnO2@TiO2 is 400e600 nm much larger than that of polysaccharide, indicating that the SnO2 and TiO2 have been successfully deposited on the surface of polysaccharide. After the polysaccharide template was removed by calcination in the air atmosphere, the structure of SnO2@TiO2 was obtained as shown in Fig. 1c. The core-shell structure can be clearly observed in SnO2@TiO2 composite. In addition, it can be found from Fig. 1c that the SnO2@TiO2 composite is consisted of many nanoparticles, meaning that this composite may own a porous structure. Then, the novel structure was further confirmed by TEM characterization, as shown in Fig. 1d. It can be obviously found from the TEM images that the composite has a core-shell structure. Moreover, the yolk-like structure of SnO2@TiO2 composites is also effectively verified by corresponding TEM elemental mapping results (Fig. 2). From the mapping result, the different distribution of SnO2 and TiO2 can be clearly distinguished. The presence of the TiO2 is confined to the shell area as well as the most of the SnO2 is confined to the core area, means that the shell is composed of TiO2 and tiny SnO2, and the dominating SnO2 locates at the core. The ICP analysis reveals the weight ratio of Sn: Ti is 0.86, indicates that the weight percentage of SnO2 in the corresponding composite is about 41.2%. The crystal structures of SnO2@TiO2 nanospheres were confirmed by X-ray diffraction (XRD) characterization, as shown in Fig. 3a. All the intensive peaks in XRD patterns can be well indexed
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Fig. 1. SEM images: (a) polysaccharide; (b) polysaccharide@SnO2@TiO2; (c) SnO2@TiO2. TEM image: (d) SnO2@TiO2; (e) the morphology of SnO2@TiO2 after 500 charge/discharge cycles at 800 mA g1.
Fig. 2. EDX elemental mappings of SnO2@TiO2.
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Fig. 3. (a) XRD patterns of SnO2@TiO2; (b) N2 adsorption/desorption isotherm and the pore size distribution (inset) of SnO2@TiO2; The XPS high-resolution spectra of the (c) Ti2p, (d) Sn3d, (e) O1s regions for the as-prepared SnO2@TiO2.
to rutile SnO2 marked by squares (JCPDS 41-1445, space group: P42/ mnm) and anatase TiO2 marked by circles (JCPDS 21-1272, space group: I41/amd) [37,38]. The pore structure of SnO2@TiO2 sample is investigated by N2 sorption measurement (Fig. 3b). The type of N2 adsorption/desorption isotherm indicates the SnO2@TiO2 has characteristics of mesoporous materials. The BET surface area is 52.8 m2 g1, and the average pore volume of BJH is 0.22 cm3 g1, which further proves the mesoporous structure of SnO2@TiO2 sample. Moreover, the identified porosity may contribute to improve the electrochemical performance of electrode by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electrons and lithium-ions [33]. The XPS analysis of SnO2@TiO2 composite was performed from 0 to 1200 eV (Fig. 3). As shown in Fig. 3c, the Ti2p spectrum for SnO2@TiO2 composite includes two obvious peaks with binding energies (BEs) of 458.89 eV and 464.61 eV, which correspond to the Ti 2p3/2 and Ti 2p1/2 core level binding energies of Ti4þ in TiO2, respectively [39,40]. The separation between these two peaks is 5.72 eV, slightly larger than the energy splitting reported for TiO2, which may be due to some SnO2 encapsulated into TiO2 nanocrystals. Similarly, as exhibited in Fig. 3d, there are two symmetrical peaks with BEs at 487.20 eV and 495.46 eV, which correspond to Sn3d5/2 and Sn3d3/2 core level binding energies of Sn4þ in SnO2, respectively, as well as the separation between them (8.26 eV) is in good agreement with the energy splitting reported for SnO2 [41e43]. As for the O1s spectrum (Fig. 3e), a portion could come from TiO2, as supported by the O1s BE peak at ~529.90 eV, while the peak at 530.40 eV may be owing to the SnO2 in the composite [26]. Based on the characterization results of XRD, FE-SEM, TEM and XPS, the facile fabrication of a uniquely porous composite with a SnO2 nanocrystal self-assembled core and a porous TiO2 shell is effectively confirmed.
Importantly, the electrochemical performance of as-prepared SnO2@TiO2 composite was well evaluated as anode materials for lithium-ion batteries. Fig. 4a displays the initial three CV curves of SnO2@TiO2 at a scan rate of 0.5 mV s1 in the potential range 0.01e3.0 V. One pair of cathodic/anodic peaks located at the 0.008 V and 0.65 V can be observed for all the curves, which respectively correspond to the alloy (cathodic scan) and dealloy (anodic scan) process. The obvious peak pair at 0.70 V and 1.56 V in the first cycle is attributed to the irreversible or partially reversible reduction of SnO2 to Sn, as well as a solid electrolyte interface (SEI) layer formation [19]. Additionally, a small peak pair at 1.73 V and 2.18 V is associated with the lithium-ions trapped in the TiO2, indicating that the TiO2 shell contributes to the total capacity of SnO2@TiO2 composite [44]. Representative galvanostatic discharge-charge profiles of SnO2@TiO2 at a current density of 200 mA g1 from 0.01 to 3.0 V are presented in Fig. 4b. It can be seen that the discharge and charge capacity for the first cycle are 1553.3 and 838.1 mAh g1 (based on the total mass of SnO2@TiO2), respectively, exhibiting a Coulombic efficiency of 54%. Such large irreversible capacity loss is mainly ascribed to the formation of SEI film and Li2O (i.e. SnO2 was reduced to Sn by metal lithium) [45]. Then, the second and third curves are almost overlapped, revealing good cycling stability of SnO2@TiO2. Fig. 4c depicts the cycling performance of the as-prepared SnO2@TiO2 sample at a current density of 800 mA g1 between 0.01 and 3.0 V. For comparison, the performance of bare SnO2 is also studied under the same conditions. Syllabify, the SnO2@TiO2 composite delivers a discharge capacity of 518.2 mAh g1 after 500 cycles. And the corresponding Coulombic efficiency is shown in Fig. S4, exhibiting a good cycling stability. In contrast, however, for the bare SnO2 sample, the discharge capacity is dropped to 3% of the initial value only after 50 cycles. The poor cycling stability of
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Fig. 4. (a) Cyclic voltammogram of SnO2@TiO2 at a scan rate of 0.5 mV s1 between 0 and 3 V; (b) Initial three discharge/charge profiles of SnO2@TiO2 at 200 mA g1; (c) Cycling performances of SnO2 nanoparticles and SnO2@TiO2 at 800 mA g1; (d) Long-term cycling performance and the corresponding Columbic efficiency of SnO2@TiO2 during cycling at 2000 mA g1; (e) Rate capability of SnO2@TiO2.
bare SnO2 sample is mainly caused by the aggregation and pulverization of SnO2 [46]. The better performance of SnO2@TiO2 composite should be attributed to its distinct structure, because the well-designed hollow structure can effectively accommodate the volume expansion of SnO2 anode materials during charge/ discharge process and hence maintains the structure integrity. So, to further insight into the electrochemical performance difference between SnO2@TiO2 and SnO2, the EIS was carried out on them and
the as-obtained Nyquist plots were compared. Generally, the impedance response of LIBs can be reflected by EIS plots, in which the diameter of the depressed semicircle is correlated with the electron transfer resistance on the electrode interface, and the angled line is related to a diffusion controlled process. Fig. S1 gives the Nyquist plots of SnO2@TiO2 and SnO2 after 5 discharge/charge cycles, which are consisting of a depressed semicircle in high frequency range and an angled line in low frequency range. It can be
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clearly seen that the SnO2@TiO2 electrode exhibits a smaller diameter of the high frequency semicircle than SnO2 electrode, demonstrating improved electron transfer capability. It is suggested that the improved electrochemical performance of SnO2@TiO2 should be attributed to its unique heterogeneous nanostructure: one hand, the encapsulation of SnO2 into constitutionally stable TiO2 could effectively restrain the agglomeration among SnO2 nanoparticles, ensuring the structure stability of electrodes, as shown in Fig. 1e; on the other hand, the mesoporous structure could promote the penetration of electrolyte into SnO2@TiO2, which ensures the larger contact interface between electrolyte and SnO2, resulting in good electrochemical reaction dynamics. On the contrary, the severe agglomeration occurred in pure SnO2 electrode due to lack of a structure which can effectively buffer the huge volume variation of SnO2 and segregate the SnO2 nanocluster, as shown in Fig. S2. The severe agglomeration would not only impede the penetration of electrolyte into interior of SnO2 nanocluster interior, but also cause that the electrode loses contact with copper current collector, finally leading to the performance of SnO2 electrode fade quickly. Even the current density increases up to 2 A g1, the SnO2@TiO2 still gives a reversible capacity of 472.7 mAh g1 after 800 cycles as shown in Fig. 4d, exhibiting impressive power and life. Besides, the morphology of the SnO2@TiO2 after 500 charge/discharge cycles at 800 mA g1 was observed by TEM (Fig. 1e). Compared with the fresh SnO2@TiO2 composite (Fig. 1d), the morphology change cannot be found visibly. It is well demonstrated that the outside TiO2 shell in the SnO2@TiO2 composite can not only contribute to restrain the volume change of SnO2, but also prevent the agglomeration of SnO2 during the charge-discharge process, which results in good stability of capacities [19]. Also the mesoporous structure of SnO2@TiO2 makes contribute to improve electrochemical performance by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electron and lithium-ion. Thus, it is demonstrated that the excellent electrochemical performance of SnO2@TiO2 should be attributed to its well-designed structure. In addition, we compared the performance of SnO2@TiO2 with the reported SnO21TiO2 wire-in-tube nanostructure and SnO2@TiO2 doubleshell nanotube. The SnO21TiO2 wire-in-tube nanostructure delivered a capacity of 393.3 mAh g1 with a retention of 27.7% after 1000 cycles at a current density of 400 mA g1, and a capacity of 241.2 mAh g1 at 3200 mA g1 [28]. The SnO2@TiO2 double-shell nanotube delivered a capacity of 300 mAh g1 coupled with a retention of 50% after only 50 cycles at a current density of 800 mA g1, and 200 mAh g1 at 1500 mA g1 after 50 cycles [29]. Then, our prepared SnO2@TiO2 delivers a capacity of 518.2 mAh g1 with a retention of 33.4% after 500 cycles at a high current density of 0.8 A g1 and a high capacity of 472.7 mAh g1 at 2 A g1 after 800 cycles. Compared with SnO21TiO2 wire-in-tube and SnO2@TiO2 double-shell nanotube, our prepared SnO2@TiO2 not only exhibits comparable performance but also facile preparation nature. As is well known, the good rate property is another indispensable indicator for electrodes as advanced lithium-ion batteries. Fig. 4e illustrates the rate capability of SnO2@TiO2 at various current densities from 200 to 5000 mA g1. The specific capacities are 693.1, 588.6, 536, 480.1 and 388.9 mAh g1 when cycles at 200, 800, 1000, 2000 and 5000 mA g1, respectively. When the current density returns to 200 mA g1 after rate test, the discharge capacity can recover to 532.9 mAh g1 after extra 139 cycles. It is indicated that the as-prepared SnO2@TiO2 has good rate capability. These findings demonstrate that the SnO2@TiO2 prepared by our approach has an excellent electrochemical performance as anode materials for lithium-ion batteries. Considering the lithium insertion potential of anatase TiO2 is mainly between 1.0 and 3.0 V, the lithium storage performance of quasi-anatase TiO2 hollow spheres (TiO2 HS, the
preparation process see Supporting Information) was also studied at 200 mA g1 between 1.0 and 3.0 V to insight into the contribution of TiO2 to the specific capacity of SnO2@TiO2, as shown in Fig. S3. After 100 cycles, the TiO2 HS delivers a capacity of 144.1 mAh g1. The SnO2@TiO2 can release a high capacity of 540.1 mAh g1 after 100 cycles at 200 mA g1, as shown in Fig. 4e (from 100th to 200th cycle). Thus, the contribution of TiO2 to the specific capacity of SnO2@TiO2 is estimated to be 15.7% based on the formula of CðTiO2ÞWðTiO2Þ 100% (The C(TiO2) and C(SnO2@TiO2) is CðSnO2@TiO2Þ specific capacity of TiO2 and SnO2@TiO2 under the same test conditions, respectively; the W(TiO2) is the percentage of TiO2 in SnO2@TiO2 composite). It is further demonstrated that the TiO2 mainly works as a function of structure support in SnO2@TiO2. 4. Conclusions In summary, the yolk-like SnO2@TiO2 composite with a unique architecture of SnO2 spherical core encapsulated into a porous TiO2 shell has been fabricated by a facile and novel method, avoiding employ of common HF-etching process. When tested as the anode material for lithium-ion batteries, the SnO2@TiO2 exhibits an excellent cycling performance and high rate capacity, delivering a reversible capacity of 472.7 mAh g1 even at a high current density of 2 A g1 for up to 800 cycles. It is demonstrated that the excellent performance of SnO2@TiO2 is attributed to its unique architecture: (1) the synergistic effect of outside TiO2 shell and inner void space can not only buffer the volume change of SnO2, but also prevent the agglomeration of SnO2, ultimately resulting in a good structural stability during long-term cycling process; (2) the mesoporous structure also contributes to the improvement of electrochemical performance by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electrons and lithium-ions. This work may provide a broader vision into fabricating yolk-like SnO2@TiO2 heterostructures for high-performance anode materials of lithium-ion batteries. Acknowledgments We thank the Instrumental Analysis Center of Shanghai Jiao Tong University for Materials Characterization. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.01.253. References [1] X. Meng, X.-Q. Yang, X. Sun, Emerging applications of atomic layer deposition for lithium-ion battery studies, Adv. Mater. 24 (2012) 3589e3615. [2] Y. Li, X.H. Li, Z.X. Wang, H.J. Guo, T. Li, Distinct impact of cobalt salt type on the morphology, microstructure, and electrochemical properties of Co3O4 synthesized by ultrasonic spray pyrolysis, J. Alloys Compd. 696 (2017) 836e843. [3] Z. Xiu, D. Kim, M.H. Alfaruqi, J. Song, S. Kim, P.T. Duong, V. Mathew, J.P. Baboo, J. Kim, Ultrafine molybdenum oxycarbide nanoparticles embedded in Ndoped carbon as a superior anode material for lithium-ion batteries, J. Alloys Compd. 696 (2017) 143e149. [4] J. Yuan, C. Chen, Y. Hao, X. Zhang, R. Agrawal, W. Zhao, C. Wang, H. Yu, X. Zhu, Y. Yu, Z. Xiong, Y. Xie, Fabrication of three-dimensional porous ZnMn2O4 thin films on Ni foams through electrostatic spray deposition for high-performance lithium-ion battery anodes, J. Alloys Compd. 696 (2017) 1174e1179. [5] H. Bin, Z. Yao, S. Zhu, C. Zhu, H. Pan, Z. Chen, C. Wolverton, D. Zhang, A highperformance anode material based on FeMnO3/graphene composite, J. Alloys Compd. 695 (2017) 1223e1230. [6] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Tin-based amorphous oxide: a high-capacity lithium-ion-storage material, Science 276 (1997) 1395e3597. [7] E. Frackowiak, F. Beguin, Electrochemical storage of energy in carbon nanotubes and nanostructured carbons, Carbon 10 (2002) 1775e1787.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Impressive lithium storage of SnO2@TiO2 nanospheres with a yolk-like core derived from self-assembled SnO2 nanoparticles Qinghua Tian a, b, *, Yang Tian b, Wei Zhang a, Jun Huang b, Zhengxi Zhang b, Li Yang b, ** a b
Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2016 Received in revised form 18 January 2017 Accepted 22 January 2017 Available online 25 January 2017
A unique structure of yolk-like SnO2@TiO2 nanospheres has been fabricated via SnO2 nanoparticles selfassembled into a core during calcination process. Moreover, this as-prepared composite owns a mesoporous structure, which can accelerate the diffusion of both electrons and lithium-ions by providing a high electrode-electrolyte contact area. When evaluated as anode material for lithium-ion batteries, the SnO2@TiO2 gives a high reversible capacity of 472.7 mAh g1 even at 2 A g1 after 800 cycles, exhibiting significantly improved electrochemical performance. This work may provide a broader vision into fabricating yolk-like SnO2@TiO2 heterostructures for high-performance anode materials of lithium-ion batteries. © 2017 Elsevier B.V. All rights reserved.
Keywords: SnO2@TiO2 Nanostructures Electrochemical properties Anode Lithium-ion batteries
1. Introduction Rechargeable lithium-ion batteries (LIBs) have attracted the most widely attention for portable electronic devices and electrical/ hybrid vehicles, due to their high operating voltage, high energy density and low self-discharge rate [1e5]. Commercial graphite anodes are commonly used as the anode materials in LIBs nowadays since their high reversibility. But, their limited theoretical capacity (about 372 mAh g1) cannot meet the pressing demands of next generation LIBs [6,7]. To meet the increasing demand for LIBs with excellent rate capability and high reversible capacity, massive efforts have been undertaken to develop new electrode materials or design unique structures of electrode materials [8e11]. As one kind of the most promising candidates for anode materials, transition metal oxides have been widely investigated for LIBs anodes to replace the commercial graphite anode due to their high theoretical capacities [12]. Among them, SnO2 has been extensively studied for lithium storage due to its abundance, low onset potential and high theoretical capacity (782 mAh g1) [13,14]. However, the practical implementation of SnO2 typically undergoes severe pulverization
* Corresponding author. Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China. ** Corresponding author. E-mail addresses: [email protected] (Q. Tian), [email protected] (L. Yang). http://dx.doi.org/10.1016/j.jallcom.2017.01.253 0925-8388/© 2017 Elsevier B.V. All rights reserved.
induced by large volume expansion (up to 250%) and agglomeration during the charge-discharge process, leading to the rapid capacity fade [15]. One of the effective strategies for improving the performance of SnO2 electrodes is fabricating heterostructures of SnO2 with other stable materials [16e18]. Considering the merit of negligible volume change (<4%) upon Liþ intercalation and de-intercalation, TiO2 has been proposed as a prospective candidate for the mechanical support of SnO2 [19e25]. There is a typical strategy has been employed, namely through designing a hollow structure of SnO2/TiO2 composite with a hollow space and one-dimensional TiO2 surface coating to stabilize the SEI and accommodate the free expansion of SnO2 anode materials, such as the core-shell nanowires or double-shell nanotubes [26e29]. Although exhibited better cycling and improved rate property compared to single component, the composites with the hollow space and one-dimensional structures have been involved in the complex synthesis processes, such as the chemical vapor deposition, electrospinning processes and even the use of high concentration of alkali etching (NaOH) [26,28,29]. Thus, it is worthy of searching facile approaches to prepare heterostructures of SnO2 with TiO2. Herein, a nanostructure of yolk-like SnO2@TiO2 has been successfully fabricated by a novel and facile method, as illustrated in Scheme 1, which effectively avoids the use of HF or high concentration of alkali etching. In the as-prepared yolk-like SnO2@TiO2
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Scheme 1. Synthesis procedure of the yolk-like SnO2@TiO2 nanospheres.
composite, the TiO2 not only works as a mechanical support which can effectively restrain the pulverization of SnO2 upon Liþ intercalation and de-intercalation, but also prevents the aggregation of the SnO2 and contributes to the capacity within a certain voltage range [30]. Moreover, the inside rich void space can also accommodate the extreme volume expansion of SnO2 during chargedischarge process [31,32]. Also the mesoporous structure of the special composite makes great contribution to improve the performance by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electrons and lithium-ions [33,34]. When evaluated as the anode material for LIBs, the yolklike SnO2@TiO2 composite with a void space exhibits a high reversible capacity of 472.7 mAh g1 at a high current density of 2000 mA g1 even after 800 cycles. 2. Experimental 2.1. Preparation of SnO2@TiO2 The polysaccharide nanospheres were first prepared by hydrothermal method according to our previous work [35]. Then, polysaccharide@SnO2 nanospheres were fabricated by depositing the SnO2 on the surface of the polysaccharide. Briefly, 100 mg polysaccharide was dispersed in the mixed solution (100 ml DI water and 100 ml ethanol) by ultrasound for 20 min. Then 120 mg SnCl2$2H2O was successively added into the mixed solution under vigorous stirring. After stirring for 24 h, the product was centrifuged and dried at 60 C for 12 h. According to the work of Wang [36], the polysaccharide@SnO2@TiO2 was prepared. Typically, the polysaccharide@SnO2 nanospheres were dispersed in 20 ml of ethanol/ acetonitrile mixture (3:1 v/v) and 0.15 ml ammonia solution (32%) was then added under vigorous stirring at room temperature. After that, 0.3 ml of titanium isopropoxide (TTIP) in 5 ml ethanol/ acetonitrile (3:1 v/v) was slowly introduced into the above suspension with stirring. After stirring for 2 h, the product was recovered by centrifugation, followed by washed with DI water for two times and dried under 60 Cfor 12 h. Finally, the SnO2@TiO2 particles were obtained by as-collected product calcination at 550 Cfor 5 h in air. It is worth noting that SnO2 nanoparticles deposited on surface of polysaccharide self-assembles into a core to reduce the whole surface energy during calcination process, finally resulting in formation of yolk-like SnO2@TiO2 nanospheres. For comparison, the SnO2 nanoparticles were also prepared by polysaccharide@SnO2 composite calcination at 550 C in air for 5 h. 2.2. Characterizations The morphologies and microstructures of the products were studied using field emitting scanning electron microscopy (FE-SEM, JEOL JSM-7401F) and transmission electron microscopy (TEM, JEOL JEM-2010) with an energy dispersive X-ray spectrometer (EDX). The crystal structure and composition were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-Rbusing Cu Ka radiation) and Inductively Coupled Plasma Mass Spectrometer (ICP-
MS, Agilent 7500a). X-Ray photoelectron spectroscopy (XPS) experiments were carried out on an AXIS UltraDLD instrument. The surface area and pore volume of the prepared materials were characterized by Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) methods based on the Surface Area and Porosimetry Analyzer (ASAP 2010 MþC). Electrochemical measurements were performed using 2016type coin cells assembled in an argon-filled glove box (German, M. Braun Co., [O2]< 1 ppm, [H2O] < 1 ppm). For preparing working electrodes, a mixture of the active material, acetylene black, and polyvinylidene fluoride (PVDF) binder at a weight ratio of 80: 10: 10 was pasted on pure copper foil. Pure lithium foil was used as the counter electrode. A glass fiber (GF/A) from Whatman was used as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (EC þDMC) (1: 1 in volume). The cells were cycled under 800 and 2000 mA g1 between cutoff voltages of 3.0 and 0.01 V on a CT2001A cell test instrument (LAND Electronic Co.) at room temperature. Cyclic voltammetry (CV) was implemented on a CHI660D electrochemical workstation. Electrochemical impedance spectrum (EIS) measurements were performed using a CHI660D electrochemical workstation in the frequency range from 100 KHz to 0.01 Hz with an ac perturbation of 5.0 mV s1.
3. Results and discussion The morphology and structure of the yolk-like SnO2@TiO2 composite were examined by the FESEM and TEM/EDX. As shown in Fig. 1a, the diameter of polysaccharide is 200e300 nm, and with a smooth surface. The morphology of polysaccharide@SnO2@TiO2 is exhibited in Fig. 1b. The diameter of polysaccharide@SnO2@TiO2 is 400e600 nm much larger than that of polysaccharide, indicating that the SnO2 and TiO2 have been successfully deposited on the surface of polysaccharide. After the polysaccharide template was removed by calcination in the air atmosphere, the structure of SnO2@TiO2 was obtained as shown in Fig. 1c. The core-shell structure can be clearly observed in SnO2@TiO2 composite. In addition, it can be found from Fig. 1c that the SnO2@TiO2 composite is consisted of many nanoparticles, meaning that this composite may own a porous structure. Then, the novel structure was further confirmed by TEM characterization, as shown in Fig. 1d. It can be obviously found from the TEM images that the composite has a core-shell structure. Moreover, the yolk-like structure of SnO2@TiO2 composites is also effectively verified by corresponding TEM elemental mapping results (Fig. 2). From the mapping result, the different distribution of SnO2 and TiO2 can be clearly distinguished. The presence of the TiO2 is confined to the shell area as well as the most of the SnO2 is confined to the core area, means that the shell is composed of TiO2 and tiny SnO2, and the dominating SnO2 locates at the core. The ICP analysis reveals the weight ratio of Sn: Ti is 0.86, indicates that the weight percentage of SnO2 in the corresponding composite is about 41.2%. The crystal structures of SnO2@TiO2 nanospheres were confirmed by X-ray diffraction (XRD) characterization, as shown in Fig. 3a. All the intensive peaks in XRD patterns can be well indexed
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Fig. 1. SEM images: (a) polysaccharide; (b) polysaccharide@SnO2@TiO2; (c) SnO2@TiO2. TEM image: (d) SnO2@TiO2; (e) the morphology of SnO2@TiO2 after 500 charge/discharge cycles at 800 mA g1.
Fig. 2. EDX elemental mappings of SnO2@TiO2.
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Fig. 3. (a) XRD patterns of SnO2@TiO2; (b) N2 adsorption/desorption isotherm and the pore size distribution (inset) of SnO2@TiO2; The XPS high-resolution spectra of the (c) Ti2p, (d) Sn3d, (e) O1s regions for the as-prepared SnO2@TiO2.
to rutile SnO2 marked by squares (JCPDS 41-1445, space group: P42/ mnm) and anatase TiO2 marked by circles (JCPDS 21-1272, space group: I41/amd) [37,38]. The pore structure of SnO2@TiO2 sample is investigated by N2 sorption measurement (Fig. 3b). The type of N2 adsorption/desorption isotherm indicates the SnO2@TiO2 has characteristics of mesoporous materials. The BET surface area is 52.8 m2 g1, and the average pore volume of BJH is 0.22 cm3 g1, which further proves the mesoporous structure of SnO2@TiO2 sample. Moreover, the identified porosity may contribute to improve the electrochemical performance of electrode by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electrons and lithium-ions [33]. The XPS analysis of SnO2@TiO2 composite was performed from 0 to 1200 eV (Fig. 3). As shown in Fig. 3c, the Ti2p spectrum for SnO2@TiO2 composite includes two obvious peaks with binding energies (BEs) of 458.89 eV and 464.61 eV, which correspond to the Ti 2p3/2 and Ti 2p1/2 core level binding energies of Ti4þ in TiO2, respectively [39,40]. The separation between these two peaks is 5.72 eV, slightly larger than the energy splitting reported for TiO2, which may be due to some SnO2 encapsulated into TiO2 nanocrystals. Similarly, as exhibited in Fig. 3d, there are two symmetrical peaks with BEs at 487.20 eV and 495.46 eV, which correspond to Sn3d5/2 and Sn3d3/2 core level binding energies of Sn4þ in SnO2, respectively, as well as the separation between them (8.26 eV) is in good agreement with the energy splitting reported for SnO2 [41e43]. As for the O1s spectrum (Fig. 3e), a portion could come from TiO2, as supported by the O1s BE peak at ~529.90 eV, while the peak at 530.40 eV may be owing to the SnO2 in the composite [26]. Based on the characterization results of XRD, FE-SEM, TEM and XPS, the facile fabrication of a uniquely porous composite with a SnO2 nanocrystal self-assembled core and a porous TiO2 shell is effectively confirmed.
Importantly, the electrochemical performance of as-prepared SnO2@TiO2 composite was well evaluated as anode materials for lithium-ion batteries. Fig. 4a displays the initial three CV curves of SnO2@TiO2 at a scan rate of 0.5 mV s1 in the potential range 0.01e3.0 V. One pair of cathodic/anodic peaks located at the 0.008 V and 0.65 V can be observed for all the curves, which respectively correspond to the alloy (cathodic scan) and dealloy (anodic scan) process. The obvious peak pair at 0.70 V and 1.56 V in the first cycle is attributed to the irreversible or partially reversible reduction of SnO2 to Sn, as well as a solid electrolyte interface (SEI) layer formation [19]. Additionally, a small peak pair at 1.73 V and 2.18 V is associated with the lithium-ions trapped in the TiO2, indicating that the TiO2 shell contributes to the total capacity of SnO2@TiO2 composite [44]. Representative galvanostatic discharge-charge profiles of SnO2@TiO2 at a current density of 200 mA g1 from 0.01 to 3.0 V are presented in Fig. 4b. It can be seen that the discharge and charge capacity for the first cycle are 1553.3 and 838.1 mAh g1 (based on the total mass of SnO2@TiO2), respectively, exhibiting a Coulombic efficiency of 54%. Such large irreversible capacity loss is mainly ascribed to the formation of SEI film and Li2O (i.e. SnO2 was reduced to Sn by metal lithium) [45]. Then, the second and third curves are almost overlapped, revealing good cycling stability of SnO2@TiO2. Fig. 4c depicts the cycling performance of the as-prepared SnO2@TiO2 sample at a current density of 800 mA g1 between 0.01 and 3.0 V. For comparison, the performance of bare SnO2 is also studied under the same conditions. Syllabify, the SnO2@TiO2 composite delivers a discharge capacity of 518.2 mAh g1 after 500 cycles. And the corresponding Coulombic efficiency is shown in Fig. S4, exhibiting a good cycling stability. In contrast, however, for the bare SnO2 sample, the discharge capacity is dropped to 3% of the initial value only after 50 cycles. The poor cycling stability of
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Fig. 4. (a) Cyclic voltammogram of SnO2@TiO2 at a scan rate of 0.5 mV s1 between 0 and 3 V; (b) Initial three discharge/charge profiles of SnO2@TiO2 at 200 mA g1; (c) Cycling performances of SnO2 nanoparticles and SnO2@TiO2 at 800 mA g1; (d) Long-term cycling performance and the corresponding Columbic efficiency of SnO2@TiO2 during cycling at 2000 mA g1; (e) Rate capability of SnO2@TiO2.
bare SnO2 sample is mainly caused by the aggregation and pulverization of SnO2 [46]. The better performance of SnO2@TiO2 composite should be attributed to its distinct structure, because the well-designed hollow structure can effectively accommodate the volume expansion of SnO2 anode materials during charge/ discharge process and hence maintains the structure integrity. So, to further insight into the electrochemical performance difference between SnO2@TiO2 and SnO2, the EIS was carried out on them and
the as-obtained Nyquist plots were compared. Generally, the impedance response of LIBs can be reflected by EIS plots, in which the diameter of the depressed semicircle is correlated with the electron transfer resistance on the electrode interface, and the angled line is related to a diffusion controlled process. Fig. S1 gives the Nyquist plots of SnO2@TiO2 and SnO2 after 5 discharge/charge cycles, which are consisting of a depressed semicircle in high frequency range and an angled line in low frequency range. It can be
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clearly seen that the SnO2@TiO2 electrode exhibits a smaller diameter of the high frequency semicircle than SnO2 electrode, demonstrating improved electron transfer capability. It is suggested that the improved electrochemical performance of SnO2@TiO2 should be attributed to its unique heterogeneous nanostructure: one hand, the encapsulation of SnO2 into constitutionally stable TiO2 could effectively restrain the agglomeration among SnO2 nanoparticles, ensuring the structure stability of electrodes, as shown in Fig. 1e; on the other hand, the mesoporous structure could promote the penetration of electrolyte into SnO2@TiO2, which ensures the larger contact interface between electrolyte and SnO2, resulting in good electrochemical reaction dynamics. On the contrary, the severe agglomeration occurred in pure SnO2 electrode due to lack of a structure which can effectively buffer the huge volume variation of SnO2 and segregate the SnO2 nanocluster, as shown in Fig. S2. The severe agglomeration would not only impede the penetration of electrolyte into interior of SnO2 nanocluster interior, but also cause that the electrode loses contact with copper current collector, finally leading to the performance of SnO2 electrode fade quickly. Even the current density increases up to 2 A g1, the SnO2@TiO2 still gives a reversible capacity of 472.7 mAh g1 after 800 cycles as shown in Fig. 4d, exhibiting impressive power and life. Besides, the morphology of the SnO2@TiO2 after 500 charge/discharge cycles at 800 mA g1 was observed by TEM (Fig. 1e). Compared with the fresh SnO2@TiO2 composite (Fig. 1d), the morphology change cannot be found visibly. It is well demonstrated that the outside TiO2 shell in the SnO2@TiO2 composite can not only contribute to restrain the volume change of SnO2, but also prevent the agglomeration of SnO2 during the charge-discharge process, which results in good stability of capacities [19]. Also the mesoporous structure of SnO2@TiO2 makes contribute to improve electrochemical performance by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electron and lithium-ion. Thus, it is demonstrated that the excellent electrochemical performance of SnO2@TiO2 should be attributed to its well-designed structure. In addition, we compared the performance of SnO2@TiO2 with the reported SnO21TiO2 wire-in-tube nanostructure and SnO2@TiO2 doubleshell nanotube. The SnO21TiO2 wire-in-tube nanostructure delivered a capacity of 393.3 mAh g1 with a retention of 27.7% after 1000 cycles at a current density of 400 mA g1, and a capacity of 241.2 mAh g1 at 3200 mA g1 [28]. The SnO2@TiO2 double-shell nanotube delivered a capacity of 300 mAh g1 coupled with a retention of 50% after only 50 cycles at a current density of 800 mA g1, and 200 mAh g1 at 1500 mA g1 after 50 cycles [29]. Then, our prepared SnO2@TiO2 delivers a capacity of 518.2 mAh g1 with a retention of 33.4% after 500 cycles at a high current density of 0.8 A g1 and a high capacity of 472.7 mAh g1 at 2 A g1 after 800 cycles. Compared with SnO21TiO2 wire-in-tube and SnO2@TiO2 double-shell nanotube, our prepared SnO2@TiO2 not only exhibits comparable performance but also facile preparation nature. As is well known, the good rate property is another indispensable indicator for electrodes as advanced lithium-ion batteries. Fig. 4e illustrates the rate capability of SnO2@TiO2 at various current densities from 200 to 5000 mA g1. The specific capacities are 693.1, 588.6, 536, 480.1 and 388.9 mAh g1 when cycles at 200, 800, 1000, 2000 and 5000 mA g1, respectively. When the current density returns to 200 mA g1 after rate test, the discharge capacity can recover to 532.9 mAh g1 after extra 139 cycles. It is indicated that the as-prepared SnO2@TiO2 has good rate capability. These findings demonstrate that the SnO2@TiO2 prepared by our approach has an excellent electrochemical performance as anode materials for lithium-ion batteries. Considering the lithium insertion potential of anatase TiO2 is mainly between 1.0 and 3.0 V, the lithium storage performance of quasi-anatase TiO2 hollow spheres (TiO2 HS, the
preparation process see Supporting Information) was also studied at 200 mA g1 between 1.0 and 3.0 V to insight into the contribution of TiO2 to the specific capacity of SnO2@TiO2, as shown in Fig. S3. After 100 cycles, the TiO2 HS delivers a capacity of 144.1 mAh g1. The SnO2@TiO2 can release a high capacity of 540.1 mAh g1 after 100 cycles at 200 mA g1, as shown in Fig. 4e (from 100th to 200th cycle). Thus, the contribution of TiO2 to the specific capacity of SnO2@TiO2 is estimated to be 15.7% based on the formula of CðTiO2ÞWðTiO2Þ 100% (The C(TiO2) and C(SnO2@TiO2) is CðSnO2@TiO2Þ specific capacity of TiO2 and SnO2@TiO2 under the same test conditions, respectively; the W(TiO2) is the percentage of TiO2 in SnO2@TiO2 composite). It is further demonstrated that the TiO2 mainly works as a function of structure support in SnO2@TiO2. 4. Conclusions In summary, the yolk-like SnO2@TiO2 composite with a unique architecture of SnO2 spherical core encapsulated into a porous TiO2 shell has been fabricated by a facile and novel method, avoiding employ of common HF-etching process. When tested as the anode material for lithium-ion batteries, the SnO2@TiO2 exhibits an excellent cycling performance and high rate capacity, delivering a reversible capacity of 472.7 mAh g1 even at a high current density of 2 A g1 for up to 800 cycles. It is demonstrated that the excellent performance of SnO2@TiO2 is attributed to its unique architecture: (1) the synergistic effect of outside TiO2 shell and inner void space can not only buffer the volume change of SnO2, but also prevent the agglomeration of SnO2, ultimately resulting in a good structural stability during long-term cycling process; (2) the mesoporous structure also contributes to the improvement of electrochemical performance by providing a high electrode-electrolyte contact area and accelerating the diffusion of both electrons and lithium-ions. This work may provide a broader vision into fabricating yolk-like SnO2@TiO2 heterostructures for high-performance anode materials of lithium-ion batteries. Acknowledgments We thank the Instrumental Analysis Center of Shanghai Jiao Tong University for Materials Characterization. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.01.253. References [1] X. Meng, X.-Q. Yang, X. Sun, Emerging applications of atomic layer deposition for lithium-ion battery studies, Adv. Mater. 24 (2012) 3589e3615. [2] Y. Li, X.H. Li, Z.X. Wang, H.J. Guo, T. Li, Distinct impact of cobalt salt type on the morphology, microstructure, and electrochemical properties of Co3O4 synthesized by ultrasonic spray pyrolysis, J. Alloys Compd. 696 (2017) 836e843. [3] Z. Xiu, D. Kim, M.H. Alfaruqi, J. Song, S. Kim, P.T. Duong, V. Mathew, J.P. Baboo, J. Kim, Ultrafine molybdenum oxycarbide nanoparticles embedded in Ndoped carbon as a superior anode material for lithium-ion batteries, J. Alloys Compd. 696 (2017) 143e149. [4] J. Yuan, C. Chen, Y. Hao, X. Zhang, R. Agrawal, W. Zhao, C. Wang, H. Yu, X. Zhu, Y. Yu, Z. Xiong, Y. Xie, Fabrication of three-dimensional porous ZnMn2O4 thin films on Ni foams through electrostatic spray deposition for high-performance lithium-ion battery anodes, J. Alloys Compd. 696 (2017) 1174e1179. [5] H. Bin, Z. Yao, S. Zhu, C. Zhu, H. Pan, Z. Chen, C. Wolverton, D. Zhang, A highperformance anode material based on FeMnO3/graphene composite, J. Alloys Compd. 695 (2017) 1223e1230. [6] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Tin-based amorphous oxide: a high-capacity lithium-ion-storage material, Science 276 (1997) 1395e3597. [7] E. Frackowiak, F. Beguin, Electrochemical storage of energy in carbon nanotubes and nanostructured carbons, Carbon 10 (2002) 1775e1787.
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