Journal of Alloys and Compounds 688 (2016) 908e913
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Synthesis of SnO2/Sn hybrid hollow spheres as high performance anode materials for lithium ion battery Ruiping Liu a, b, *, Weiming Su a, Peng He a, Chao Shen a, Chao Zhang a, Fabing Su c, Chang-An Wang d a
Department of Materials Science and Engineering, China University of Mining & Technology (Beijing), Beijing, 100083, China State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology (Beijing), Beijing, 100083, China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, CAS, Beijing, 100190, China d School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing, 100084, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2016 Received in revised form 12 July 2016 Accepted 18 July 2016 Available online 27 July 2016
The hollow SnO2 spheres were synthesized by using carbon spheres as hard templates, and then carboncoated SnO2/Sn hybrid hollow spheres can be obtained through annealing process after carbon coating. The as-prepared hybrid spheres are well dispersed and coated with a uniform carbon layer. The carboncoated SnO2/Sn hybrid hollow spheres electrode exhibits high initial Coulombic efﬁciency (82.3%), high reversible capacity and good cycling stability (the initial charge and discharge capacity of 878.7 and 1061.6 mA h/g was achieved and the discharge capacities maintained 401 mA h/g after 50 cycles at 0.1 C) and good rate performance (1156.8 mA h/g at 0.1 C, 752.2 mA h/g at 0.2 C, 481.4 mA h/g at 0.5 C, 289.5 mA h/g at 1 C, and 120.6 mA h/g at 2 C, and more importantly, when the current density ﬁnally backs to 0.1 C, a capacity of 811.6 mA h/g can be restored) for anode materials in lithium ion batteries due to the hierarchical porous structure, carbon coating and the presence of Sn. © 2016 Elsevier B.V. All rights reserved.
Keywords: SnO2 Hybrid materials Hollow spheres Anode materials Lithium ion battery
1. Introduction Lithium ion batteries (LIBs) are considered as the most promising rechargeable batteries due to their relatively high energy density, good cycle life and power performance [1e3]. The commercial graphite-based anode materials show the limited gravimetric capacity (372 mA h/g), therefore, an alternative anode material with high performance is highly demanded. As a typical ntype semi-conductor with a wide band gap, tin and tin dioxide (SnO2) has attracted great attention as anode materials for lithiumion battery due to the capability of reversibly forming alloys of Sn with lithium [4,5]. However, the large volume changes (>300%) during the lithiation and de-lithiation process could bring cracking, crumbling and even peeling off from the current collector, which results in the capacity decay and poor cycling life of Sn-based anode materials [6,7]. Generally, there are two main routes to solve the problem, that is, design of the micro- and nanostructures and combination of carbon with tin oxide. In order to alleviate the
* Corresponding author. Department of Materials Science and Engineering, China University of Mining & Technology (Beijing), Beijing, 100083, China. E-mail address: [email protected]
(R. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.07.194 0925-8388/© 2016 Elsevier B.V. All rights reserved.
volume effect during the charge and discharge process, various nanostructure has been prepared in past two decades, such as zerodimensional (0D) nanoparticles [8,9], one-dimensional (1D) nanorods , nanobelts , nanowires , nanotubes , nanoﬁber , two-dimensional (2D) nanosheets , threedimensional (3D) hierarchical hollow nanospheres , and mesoporous structures . Recently, SnO2 hollow spheres with high surface area, high loading capacity and low density have received great attention because of its large surface area and porous structure which could accommodate the volume change during the charge and discharge process. Until now, the general approach for the synthesis of hollow spheres involves the hard templates method such as polymer, silica and carbon which can be removed by calcination or chemical etching  and soft templates method such as hydrothermal synthesis by Oswald ripening process . However, hollow spheres prepared by hydrothermal method usually suffer from different experimental disadvantages related to long process and the difﬁculty of size controlling. By comparison, template-assisted method has notable advantages such as facile procedure and low cost. Up to now, SnO2 spheres with hollow structure have been synthesized via various method, and the electrochemical properties
R. Liu et al. / Journal of Alloys and Compounds 688 (2016) 908e913
were also investigated intensively. However, most researches put their attention on the synthesis of tin dioxide hollow spheres with controllable structure, and the as-obtained hollow SnO2 spheres show the lower initial Coulombic efﬁciency due to the irreversible reduction reaction between the Li and SnO2 during the charging process . Thus, it is highly desired to synthesize Sn-based anode materials with high initial Coulombic efﬁciency, better cycle and rate performance. Herein, we developed a facile strategy to generate carboncoated SnO2/Sn hybrid hollow spheres with good dispersion and uniform structure, and the electrochemical properties of the obtained SnO2/Sn hybrid spheres have been investigated.
hollow spheres), carbon black and Polyvinylidene Fluoride (PVDF) in a weight ratio of 80: 10: 10. The neat lithium metal foil was used as counter electrode and polypropylene microporous ﬁlm (Celgard 2400) was used as separator. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC)/ dimethyl carbonate (DMC) (1:1:1 b y volume). The cells were cycled between 0.001 and 2 V (vs. Li/Liþ) at a current density of 0.1Ce2 C at room temperature. The cyclic voltammograms (CV) and electrochemical impedance spectrum (EIS) were carried out on an electrochemical workstation (CHI660C, Shanghai Chenhua). The CV was performed at a scan rate of 0.1 mV/s. EIS measurements were recorded over the frequency range from 100 kHz to 0.01 Hz.
3. Result and discussion
2.1. Synthesis of SnO2 hollow spheres
XRD patterns shown in Fig. 1 indicate that the presence of SnO2 after calcination under different temperature. It can be seen that after the calcination of carbon/tin composite spheres at 500 C, the tin precursor was transformed into cassiterite SnO2, and with increasing temperature, the crystallization degree increased. But when the calcination temperature reach to 800 C, XRD peaks become rough and the crystallization degree decreased. The crystal size of SnO2 can be calculated by applying (110) diffraction in the Scherrer equation D ¼ kl/b cosq (where k is a shape factor, about 0.89, l is the X-ray wavelength, b is the FWHM of the diffraction line, and q is the diffraction angle). According to the calculation results, the crystal size of SnO2 calcined at 600 C was 9.5 nm. Fig. 2 shows the XRD pattern of carbon-coated SnO2/Sn hybrid hollow spheres, which indicates the existence of Sn and SnO2. It can be deduced that the slight amounts of SnO2 was reduced to Sn by carbon which was converted from poly-dopamine during calcination. Fig. 3(a) shows the SEM image of as-synthesized carbon spheres, it can be seen that the carbon spheres are well dispersed with a uniform diameter of nearly 500e600 nm, after alkali treatment, SnCl4 solution coating and calcination, the SnO2 hollow spheres with spherical shape and monodispersity were obtained (Fig. 3 (b)). The TEM image of the inset of Fig. 3(c) shows a clear contrast between the edge and the center, indicating that they exhibited a hollow structure with the shell thickness of 15 nm. In order to increase the electric conductivity of the spheres and alleviate the
2.2. Synthesis of carbon-coated SnO2/Sn hybrid hollow spheres Typically, 300 mg as-prepared SnO2 was dispersed in Tris (50 mL, pH ¼ 8.5) solution by ultrasonication for 1 h to form a uniform suspension. After that, 50 mg dopamine hydrochloride was added to the mixture under magnetic stirring. The mixture was subjected to continuous magnetic stirring at room temperature for 12 h. Afterwards, the precipitate, [email protected]
, was collected by centrifugation and the precipitate was washed thoroughly with ethanol and deionized water before being dried at 60 C for 6 h. The obtained powder was then annealed at 700 C for 4 h under Ar to convert the poly-dopamine into carbon, and at the same time, the as-obtained SnO2 was partially reduced to Sn. The obtained composite was named as carbon-coated SnO2/Sn hybrid hollow spheres. 2.3. Characterization and electrochemical measurement The as-obtained powders were characterized with XRD (Bruker D8-Advance diffractometer), TEM (JEOL JEM-2100, operated at 200 kV) and SEM (Zeiss Supra 40 FE). BET-surface area and pore size distribution was measured by N2 adsorption at liquid nitrogen temperature using a NOVA4000 automated gas sorption system. Electrochemical tests were performed by using CR2032 cointype cells which assembled in a glove box (Mikrouna, Shanghai, China) ﬁlled with high purity Ar. The working electrodes were fabricated by mixing the active material (SnO2/[email protected]
In a typical process, 8 g of glucose (Beijing Chemical Reagent Factory, analytic purity) was dissolved in water (90 mL) to form a clear solution. The solution was then sealed in a 100 mL teﬂon-lined autoclave and maintained at 180 C for 12 h. The black or puce products were centrifuged, washed, and re-dispersed in water and ethanol for ﬁve times. The spheres were then oven-dried at 80 C overnight. Then, carbon spheres were re-dispersed and reﬂuxed at 85 C for 4 h in 1 mol/L NaOH solution to modify the functional groups on surface. After the reaction, the solution was cooled to room temperature. The black suspension and precipitate were dispersed by centrifugation, washed with water and absolute alcohol three times, and then dried at 80 C. The typical synthetic procedure for SnO2 porous core-shell spheres is as follows: The prepared carbon spheres were washed, re-dispersed in 0.2 mol/L SnCl4 solution and ultrosonicated for 30 min. The mixture was stirred for 24 h, and then the precipitate was washed thoroughly with ethanol and deionized water to remove the other impurities. Finally the SnO2 hollow spheres were obtained by heating the product at 400e800 C for 4 h in a tube furnace in air to remove the carbon template.
2 Theta(°) Fig. 1. XRD patterns of precursor after calcination under different temperature.
R. Liu et al. / Journal of Alloys and Compounds 688 (2016) 908e913
140 120 100
Absorbed volume (cc/g,STP)
Pore diameter (nm)
40 20 0 0.00
2 Theta (°) Fig. 2. XRD patterns of carbon-coated SnO2/Sn hybrid hollow spheres.
Fig. 4. Nitrogen adsorption-desorption isotherm of carbon-coated SnO2/Sn hybrid hollow spheres.
Fig. 3. SEM images of carbon spheres (a), SnO2 coated carbon spheres before (b) and after (c) calcination, carbon ecoated SnO2/Sn hybrid hollow spheres (d), TEM images of carbon ecoated SnO2/Sn hybrid hollow spheres (eef), the inset of (c) is the TEM image of SnO2 hollow spheres.
volume effect during the charge-discharge process, a uniform thin carbon layer was successfully coated on the surface of as-obtained hollow SnO2 spheres, as shown in Fig. 3 (d). The thickness of carbon layer is around 20 nm, which can improve the cycle and rate performance of the anode materials. HRTEM image presented in Fig. 3 (f) displays the lattice fringes with a spacing of 0.335 nm and 0.28 nm, which can be indexed as (110) and (101) lattice planes of SnO2 and Sn, respectively. The results coincide well with the XRD patterns, which demonstrated that the SnO2 particles are partially converted to Sn during the annealing process after carbon coating. It can be revealed that the presence of Sn will improve the initial Coulombic efﬁciency of the electrode by reducing the Liþ consuming for the conversion of SnO2 to Sn during the charge process. Fig. 4 presents the Nitrogen absorption-desorption isotherm of carbon-coated SnO2/Sn hybrid hollow spheres. It can be seen that the N2 adsorption-desorption isotherm exhibits a distinct large hysteresis loop, which is the typical characteristic of mesoporous
materials, namely a type IV isotherm with H1-shaped hysteresis loop assigned to mesoporous structure. The hierarchical porous structure of both hollow core and mesopore distributed on the shell may beneﬁt for the lithium ion diffusion process by shorten the diffusion distance due to the relative large contact area between the anode materials and liquid electrolyte. Meanwhile, the unique porous structure could also alleviate the volume effect during the charge-discharge process. The initial three CV curves of the carbon-coated SnO2/Sn hybrid hollow spheres at a scan of 0.1 mV/s1 from 0.0 to 2.0 V are depicted in Fig. 5(a). It can be seen clearly that the ﬁrst charge-discharge circle is distinguished from the other two circles. A reduction peak located at about 0.61 V is observed in the ﬁrst discharge process and disappeared in the subsequent cycles, which can be attributed to the formation of solid electrolyte interface (SEI) layer on the surface of the active materials  and the reduction of SnO2 to Sn . The ﬁrst pair shown at the potentials of 0.51 V and 0.12 V can be attributed to the alloying (cathodic scan) and de-alloying
R. Liu et al. / Journal of Alloys and Compounds 688 (2016) 908e913 0.002
(b) 1st Disharge 1st Charge 2nd Discharge 2nd Charge 25thDischarge 25thCharge 50hDischarge 50hCharge 75hDischarge 75hCharge
0.000 1st cycle 2nd cycle 3rd cycle -0.002
Potential vs Li/Li (V)
Specific Capacity (mAh/g)
Charge Discharge Efficiency
Specific Capacity (mAh/g)
20 30 Cycle Number
140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
Charge Discharge Efficiency 0.1C
1400 1200 1000
Specific Capacity (mAh/g)
30 40 Cycle Number
Fig. 5. CV curves of carbon-coated SnO2/Sn hybrid hollow spheres electrode at a voltage of 0e2 V and scan rate of 0.1 mV/s (a), voltage proﬁles of carbon-coated SnO2/Sn hybrid hollow spheres electrode at a speciﬁc current of 0.1 C (b), cycle performance of carbon-coated SnO2/Sn hybrid hollow spheres electrode at a speciﬁc current of 0.1 C and 1 C (c), rate performance of carbon-coated SnO2/Sn hybrid hollow spheres at charge/discharge rates from 0.1 C to 2 C (d).
Fig. 5(c) exhibits the galvanostatic cycle behaviors of carboncoated SnO2/Sn hybrid hollow spheres at a speciﬁc current of 0.1 C, together with their Coulombic efﬁciency. It can be seen that the hollow spheres exhibit higher initial charge higher initial charge and discharge capacity, delivering a discharge and charge capacities of 1061 and 878 mA h/g, and the capacity slightly
800 SnO2 [email protected]
(anodic scan) process . The second pair of cathodic/anodic peaks are around 0.7 V and 1.1 V, respectively. The cathodic/anodic peaks around 0.7 V and 1.1 V since the ﬁrst charge process indicate a reversible transformation between SnO2 and Sn, which suggests that the reaction may be partly reversible in this case . The anodic peak located at around 1.33 V can be assigned to the partial conversion of Sn to SnO and SnO2. The galvanostatic discharge/charge voltage proﬁles of the carbon-coated SnO2/Sn hybrid hollow spheres at a current density of 0.1 C between 0.01 and 2 V for the 1st, 2nd, 26th, 60th and 76th cycle are presented in Fig. 5 (b). In the initial discharge curve, a smooth slope in the potential range of 1.24e0.86 V could be attributed to the irreversible reduction of SnO2 to Sn [25,26]. The charge and discharge capacity for the ﬁrst cycle are 878.7 and 1061.6 mA h/g, respectively, with a initial Coulombic efﬁciency of 82.3%, which is higher than that of the reported results. For example, 43.4% for SnO2 hollow microspheres , 64.3% for branched [email protected]
@C heterostructures , 55% for Hierarchical SnO2 Hollow Nanostructures , 66% for SnOx/Carbon Nano -hybrids . The high Coulombic efﬁciency is mainly attributed to the partially reduction of SnO2 by carbon during the annealing process. The initial capacity loss can be attributed to the formation of solid electrolyte interface (SEI) ﬁlm and electrolyte decomposition. As can be seen, the second discharge curve of the carboncoated SnO2/Sn hybrid hollow spheres is different from the ﬁrst one, suggesting drastic structural changes in the electrodes due to lithium-driven electrochemical reactions. The charge/discharge curves are similar from the 25th cycle to 76th cycle, which indicate that the electrodes possess good cycle performance.
Z'/ohm Fig. 6. Nyquist plots of the hollow SnO2 spheres before and after carbon coating, the inset is the equivalent circuit used to ﬁt the experimental impedance spectra.
R. Liu et al. / Journal of Alloys and Compounds 688 (2016) 908e913 Table 1 Impedance parameters for hybrid spheres obtained by ﬁtting the data shown in Fig. 6 Samples
Hollow SnO2 spheres
Carbon-coated SnO2/Sn hybrid hollow spheres
Rs (U) Rct (U) CPE W
3.981 354.4 9.01 862.2
1.773 202.1 1.644 366.5
decreases to 654 mA h/g in the ﬁrst 10 cycles and then gradually stabilized to 401 mA h/g during the following forty cycles. According to the BET results, the higher speciﬁc surface area of the hollow spheres could promote charge-discharge process of the active materials. Different from the published results, the high initial Coulombic efﬁciency of 82.3% can be obtained from the anode materials, which then promptly increases to 96.2% within four cycles and afterward stabilizes over 97.5% for the subsequent cycles. The pre-reduced Sn in the active materials could reduce the Liþ consuming for the conversion of SnO2 to Sn during the charge process, thus ﬁnally increase the initial Coulombic efﬁciency. Fig. 5(d) illustrates the rate capability of carbon-coated SnO2/Sn hybrid hollow spheres at various current rates from 0.1 C to 2 C (1C ¼ 782 mA/g), in which the given capacity values are the average taken over 10 cycles. As expected, the capacity decreases gradually as the current rate increases. The reversible capacity of the electrode in ﬁfth cycle is 1156.8, 752.2, 481.4, 289.5, and 120.6 mA h/g when cycles at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. More importantly, when the current density ﬁnally backs to 0.1 C, a capacity of 811.6 mA h/g can be restored, conﬁrming the good rate performance and stability of the carbon-coated SnO2/Sn hybrid hollow spheres. In order to further classify the potential reason of the enhanced electrochemical properties of the carbon-coated SnO2/Sn hybrid hollow spheres electrode, electrochemical impedance spectra (EIS) were implemented on the prepared hollow SnO2 spheres before and after carbon coating. As shown in Fig. 6, the EIS spectra present in the form of Nyquist plots composed of a semicircle at the highmedium frequency region which can be attributed to the charge transfer reaction at the interface between electrode and electrolyte, and an inclined line in the low frequency region corresponds to the lithium-ion diffusion in the solid electrode . The Nyquist plots are ﬁtted by using the equivalent circuit modeling . In the equivalent circuit (inset), Rs and Rct are the ohmic resistance and the charge transfer resistance of the electrodes, and the constant phase-angle element (CPE) and the Warburg impedance (W) reﬂect the Liþ diffusion into the bulk of the active materials. It can be seen that the semicircle at the high-medium frequency region of the carbon-coated SnO2/Sn hybrid hollow spheres is smaller than that of hollow SnO2 spheres, which veriﬁes the improved electronic conductivity of prepared carbon-coated SnO2/Sn hybrid hollow spheres electrode, and thus the rate properties can be improved. According to the ﬁtting results (Table 1), both the Rs, Rct and W of carbon-coated SnO2/Sn hybrid hollow spheres are smaller than those of hollow SnO2 spheres. This result suggests that both the electric conductivity, charge transfer kinetics and diffusion of lithium ions can be effectively improved by carbon coating process. In addition, the hierarchical porous structure of the active materials can not only alleviate the volume effect by providing space for particle expansion during charge process, but also enlarge the interface area between solid electrode and electrolyte, which could improve the electron and lithium-ion transfer properties efﬁciently. Furthermore, partially reduced Sn particles in the electrode may beneﬁt to the improvement of the initial Coulombic efﬁciency of the electrode by reducing the Liþ consuming due to the irreversible conversion of SnO2 to Sn in the ﬁrst charge/discharge cycle.
4. Conclusions In summary, we have synthesized the carbon-coated SnO2/Sn hybrid hollow spheres by using carbon spheres as hard template and demonstrated their high capacity reversibility and long-term cycling stability as anode for lithium-ion batteries. The hollow SnO2 spheres are well dispersed with a uniform carbon layer on the surface, and the SnO2 particles are partially reduced to Sn particles during the annealing process after carbon coating. The lower initial Coulombic efﬁciency, poor cycling and rate performance of SnO2 could be improved by the presence of Sn particle, hierarchical porous structure and the carbon coating. These results make the asprepared carbon-coated SnO2/Sn hybrid hollow spheres a promising anode for lithium-ion batteries. Acknowledgments The authors would like to thank the ﬁnancial support from the National Natural Science Foundation of China (NSFCeNo. 51202117 and 51572145), Natural Science Foundation of Beijing (No.2162037), the funding of Beijing outstanding talent (No.2015000020124G121), the Fundamental Research Funds for the Central Universities (No.2014QJ02) and the funding of State key laboratory of Coal Resources and Safe Mining (No.SKLCRSM16KFB04). References  B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 9 (2010) 2419e2430.  M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 6861 (2001) 359e367.  Y. Li, J. Song, J. Yang, A review on structure model and energy system design of lithium-ion battery in renewable energy vehicle, Renew. Sustain. Energy Rev. 37 (2014) 627e633.  J.S. Chen, L.C. Yan, Y.T. Chen, et al., SnO2 nanoparticles with controlled carbon nanocoating as high-capacity anode materials for lithium-ion batteries, J. Phys. Chem. C 113 (2009) 20504e20508.  MingShan Wang, Ming Lei, ZhiQiang Wang, et al., Scalable preparation of porous micron-SnO2/C composites as high performance anode material for lithium ion battery, J. Power Sources 309 (2016) 238e244.  L. Yang, K. Chen, T. Dong, et al., One-pot synthesis of SnO2/C nanocapsules composites as anode materials for lithium-ion batteries, J. Nanosci. Nanotechnol. 16 (2016) 1768e1774.  R. Sugaya, M. Sugawa, H. Nomoto, Improved electrochemical performance of SnO2-mesoporous carbon hybrid as a negative electrode for lithium ion battery applications, Phys. Chem. Chem. Phys. 16 (2014) 6630e6640.  Lixiong Yin, Simin Chai, Feifei Wang, et al., Ultraﬁne SnO2 nanoparticles as a high performance anode material for lithium ion battery, Ceram. Int. 42 (2016) 9433e9437.  Jia Wang, Wei-Li Song, Zhenyu Wang, Li-Zhen Fan, Yuefei Zhang, Facile fabrication of binder-free metallic tin nanoparticle/carbon nanoﬁber hybrid electrodes for lithium-ion batteries, Electrochimica Acta 153 (2015) 468e475.  X. Ji, X. Huang, J. Liu, et al., Carbon-coated SnO2 nanorod array for lithium-ion battery anode material, Nanoscale Res. Lett. 5 (2010) 649e653.  X.Y. Xue, Z.H. Chen, L.L. Xing, et al., SnO2/a-MoO3 core-shell nanobelts and their extraordinarily high reversible capacity as lithium-ion battery anodes, Chem. Commun. 47 (2011) 5205e5207.  R. Thomas, G.M. Rao, SnO2 nanowire anchored graphene nanosheet matrix for the superior performance of Li-ion thin ﬁlm battery anode, J. Mater. Chem. A 3 (2014) 274e280.  W. Zeng, F. Zheng, R. Li, et al., Template synthesis of SnO2/a-Fe2O3 nanotube array for 3D lithium ion battery anode with large areal capacity, Nanoscale 4 (2012) 2760e2765.
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