COMMUNICATION DOI: 10.1002/chem.201303400
Ordered Mesoporous Core/Shell SnO2/C Nanocomposite as High-Capacity Anode Material for Lithium-Ion Batteries Hao Liu,[a] Sheng Chen,[b] Guoxiu Wang,*[a] and Shi Zhang Qiao*[b]
Batteries are widely considered as the best power sources for the next generation of electric vehicles (EVs) in order to relieve our reliance on non-renewable energy sources.[1] Lithium-ion batteries are considered as one of the most promising battery options for EVs, because of their highenergy density compared to conventional rechargeable batteries.[2] However, current lithium-ion batteries consisting of microsize electrode materials are not competent as highpower sources; this is attributed to the polarisation and pulverisation of microsize electrode materials operating at high current.[3] Thus, it is critical to develop new electrode materials with enhanced electrochemical performance for EVs. Tin-based oxide materials have high capacity, because a large number of lithium ions can be hosted per Sn unit to form LixSn (x 4.4) alloy.[4] To date, many nanostructured SnO2 materials, including nanorods, nanowires, nanotubes, nanosheets, nanoboxes and mesopurous SnO2, have been widely investigated as anode materials for lithium-ion batteries, because nanostructured SnO2 materials provide a short pathway for lithium-ion diffusion.[5] However, lithiation-induced volume expansion causes pulverisation of electrode materials, and hence degrades the performance and lifetime of SnO2 anode.[6] Therefore, many attempts have been made to prevent SnO2 from pulverisation, by coating SnO2 with oxides, Si and carbon.[7] In particular, carbon coating has been extensively developed because it is a cheap, abundant and low-toxic source to enhance the conductivity and stability of SnO2 anode materials.[7g–o] Recently, mesostructured materials have attracted great attention because of their unique structure and high performance for high-power energy storage.[8] Especially, mesoporous carbon-based composites have been widely investigated for high-power energy storage, by embedding active materials into pores of mesoporous carbon substrates.[9] The electrochemical property of active materials can be remarkably en[a] Dr. H. Liu, Prof. Dr. G. Wang School of Chemistry and Forensic Science University of Technology Sydney Broadway Sydney, NSW 2007 (Australia) E-mail: [email protected] [b] Dr. S. Chen, Prof. Dr. S. Z. Qiao School of Chemical Engineering The University of Adelaide, SA 5005 (Australia) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303400.
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hanced by confinement of mesoporous carbon matrix. Many mesoporous SnO2/C nanocomposites have been reported.[10] For example, Huang et al. reported a sonochemical synthesis of ordered SnO2/CMK3 composite with enhanced electrochemical performance.[10a] Lu et al. fabricated a tubular mesoporous carbon substrate (CMK5) and homogeneously dispersed SnO2 nanoparticles in the mesopore channels. The as-prepared nanocomposite exhibited high reversible capacity and fast charge/discharge kinetics at high rate.[10b] Lou et al. prepared carbon supported mesoporous SnO2 by a hydrothermal method, and achieved high capacity and good retention.[10c] However, the above-mentioned reports of mesoporous SnO2/C composites were all based on two-step synthesis, namely, production of SnO2 and carbon separately. Herein, we report a facile vacuum-assisted impregnation method to produce mesoporous SnO2/C nanocomposites with simultaneous formation of SnO2 and carbon, using mesoporous silica (SBA15) as a template and organic tin salt (C4H9SnCl3) as a tin source. This SnO2/C with high surface area and large pore volume can provide more space for electrolyte soakage and possess more sites for lithium-ion intercalation. During the high-temperature calcination, alkyl groups were carbonised and coated on the surface of crystal SnO2, to form a core/shell nanostructure. This carbon layer can enhance the conductivity of SnO2 and suppress the volume change, and hence improve its electrochemical property as anode material for lithium-ion batteries. Figure 1 a shows the small-angle X-ray diffraction (SAXRD) pattern of the highly ordered mesoporous SnO2/ C composite. The diffraction peaks at 0.98 can be indexed to the corresponding diffractions of (100) in the hexagonal P6mm space groups. The wide angle XRD pattern of mesoporous SnO2/C presented in the inset of Figure 1 a exhibits diffraction peaks between 208 and 808, which can be indexed to the standard tetragonal Cassiterite SnO2 structure (JCPDS No. 41-1455). No impurity diffraction peaks were observed, suggesting a complete transformation from organic tin precursor to crystalline tin oxide. The component of SnO2/C was confirmed by Raman microscopy (Figure 1 b). The peaks at 485 and 618 cm 1 correspond to the typical Eg and A1g vibrational modes of the Sn O bond.[11a] The bonds at 1368 and 1571 cm 1 are the typical D and G lines of carbon material.[11b] The ratio of SnO2 and carbon in composite was examined by thermogravimetric analysis (TGA; Figure 1 c). The composite lost its weight between 380 and 510 8C due to the combustion of carbon in air, which is con-
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strates 2D mesopores the same as shown in the high-resolution SEM image. The selected area electron diffraction (SAED) pattern (Figure 2 b, inset) reveals the polycrystalline structure of the mesoporous SnO2/C, which is consistent with the XRD result. Figure 2 c clearly illustrates the 2D mesoporous channels between carboncoated crystal SnO2 nanowirelike arrays. The schematic core/ shell structure is shown as the inset of Figure 2 c. The core of simulated SnO2 shows unit cells of typical rutile structure, wherein the tin atoms are sixcoordinate and the oxygen atoms three-coordinate. The high-resolution TEM image of SnO2 clearly demonstrates crysFigure 1. Characterisations of mesoporous SnO2/C. a) Small-angle XRD and wide-angle XRD (inset) patterns. talline SnO2 coated by carbon b) Raman microscopy. c) TGA and heat flow curves. d) Nitrogen sorption isotherm and corresponding pore layers (Figure 2 d). The intersize distribution (inset). planar distances of observed (110) and (101) planes are well matched with standard rutile SnO2 structure. The fast Fourisistent with an exothermal process as shown in the heat-flow curve. The carbon ratio is 13.8 %, calculated from the TGA er transformation (FFT) pattern reveals the orientations result. The porosity of the SnO2/C composite was investigatalong different directions (Figure 2 d, inset). Amorphous ed by the nitrogen sorption measurement (Figure 1 d). The nitrogen sorption isotherm of mesoporous SnO2/C demonstrates an IV type isotherm. The specific surface area is 181 m2 g 1 and pore volume is 0.22 cm3 g 1, respectively. The pore-size distribution of the mesoporous SnO2/C calculated by the Barret–Joyner–Halenda (BJH) method is presented in Figure 1 d (inset). The relatively narrow pore-size distribution at 2.9 nm is attributed to 2-dimensional (2D) hexagonal mesoporous channels in SnO2/C, replicated from the walls of silica template. The above-mentioned results clearly demonstrate that the mesoporous SnO2/C material has been successfully replicated by this vacuum-assisted method from 2D hexagonal SBA-15 silica template (SAXRD and nitrogen sorption results of SBA-15 are shown in Figures S1 and S2 in the Supporting Information). Figure 2 shows scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the mesoporous SnO2/C material. The SEM image of mesoporous SnO2/C demonstrates a large-scale rod-like SnO2/C with particle sizes of 300 nm–1 micron (Figure 2 a). The morphology of mesoporous SnO2/C is not identical to the silica template because of volume shrinkage of crystals during the hightemperature treatment.[12] The inset of Figure 2 a is a highFigure 2. a) Low-magnification SEM image of mesoporous SnO2/C particles and high-resolution SEM image of ordered wire array structure resolution SEM of the composite, which clearly shows typi(inset). b) Low-magnification TEM view of and corresponding selected cal 2D wire-like arrays, with mesopores around 3 nm (SEM area electron diffraction (SAED) pattern. c) High-resolution TEM image image of SBA-15 template is shown in Figure S3 in the Supof core/shell structured SnO2/C and its simulated scheme shown in the porting Information). Figure 2 b presents a low-magnificainset. d) Crystal lattice fringe of core/shell SnO2/C and its corresponding FFT pattern in the inset. tion TEM image of a single SnO2/C particle, which demon-
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electrode shows lower values across the real resistance (Rre) axis, demonstrating a lower overall resistance of SnO2/C electrode than uncoated SnO2 electrode, which can be attributed to the enhancement of conductivity caused by the carbon coating. Moreover, the SnO2/C electrode exhibits a lower charge-transfer resistance, as shown in moderate frequency regions, which can lead to faster lithium-ion diffusion in solid state. Galvanostatical charge/discharge tests of mesoporous SnO2 and SnO2/C were investigated at a current density of 0.1 A g 1 within the voltage range of 0.05 and 3 V for 100 cycles. The cycling performance and the Coulombic effiFigure 3. a) CV curves of mesoporous SnO2/C for the first 3 cycles, with scanning rate of 0.1 mV s 1. b) Nyquist ciencies of mesoporous SnO2 plots of mesoporous SnO2 and SnO2/C. c) Lithium storage performance of mesoporous SnO2 and SnO2/C at and SnO2/C electrodes are the galvanostatic current of 0.1 A g 1. d) Multirate tests of mesoporous SnO2 and SnO2/C, in order of 0.1, 0.2, 0.5, 1, 2, 5, 10 A g 1, and then return to low currents of 1 and 0.1 A g 1, with 5 cycles for each step. shown in Figure 3 c. The initial discharge capacities of the mesoporous SnO2 and SnO2/C electrodes are 1654 and 1696 mAh g 1, respectively. The high inicarbon layers of around 1 nm are denoted in Figure 2 d. The carbon shell will be beneficial for enhancing electronic contial discharge capacity (double the theoretical capacity) can ductivity and suppressing the volume expansion during be attributed to the two steps of lithium-ion intercalations: charge/discharge. reduction of Sn cation to Sn0 and subsequent formation of Figure 3 a shows cyclic voltammetry (CV) curves of mesoLixSn alloy, accompanied by the decomposition of non-aqueporous SnO2/C for the first 3 cycles, in the scanning region ous electrolyte. In subsequent cycles, the discharge capacity of mesoporous SnO2 electrode dramatically decreases and between 0.05 and 3 V. In the initial CV cycle, the reduction only delivers a capacity of 411 mAh g 1 after 100 cycles. In peaks at 0.8 V are attributed to the lithium insertion to SnO2 contrast, the discharge capacity of mesoporous SnO2/C after to form Sn and Li2O, accompanied by the decomposition of non-aqueous electrolyte to form a solid electrolyte inter100 cycles remained as high as 780 mAh g 1 (more than phase (SEI) layer. Lithium ions subsequently insert to Sn double commercial graphite 372 mAh g 1), indicating and carbon to form LixSn and LixC6 compounds, down to a great improvement of electrochemical performance compared to the uncoated mesoporous SnO2 (discharge curves the cut-off voltage of 0.05 V. During the anodic scanning, there are three oxidation peaks presented at 0.56, 1.25 and of mesoporous SnO2 and SnO2/C for the 1st, 2nd and 100th 1.80 V, corresponding to the oxidation stages of lithium ions cycles are available in Figure S9 in the Supporting Informaextracted from LixSn alloy, consistent with the results prevition). Moreover, the mesoporous SnO2/C electrode exhibits ously described.[9b] The CV curves of the second and third higher initial Coulombic efficiency (65.1 %) than mesoporous SnO2 (63.8 %). In the following cycles, the efficiencies cycles are identical, indicating an excellent reversibility of the mesoporous SnO2/C electrode. In contrast, CV curves of of mesoporous SnO2/C electrodes gradually increase and uncoated mesoporous SnO2 show a distinguished decrease stay at a stable level after 5 cycles, at approximately 97 to 98 %, which is higher than the uncoated mesosporous SnO2 in the intensity of the redox peaks, as well as their integrated area (Figure S8 in the Supporting Information), demon(around 96 %). The improved long-term performance can be strating a fast capacity fading and worse electrochemical attributed to the enhanced stability of the SnO2/C composite performance than the SnO2/C. The Nyquist plots of mesopopromoted by the carbon-coating layer. Multiple-current galvanostatical tests were carried out to investigate the highrous SnO2 and SnO2/C are shown in Figure 3 b. Both profiles rate performance of mesoporous SnO2 and SnO2/C (Figpresent a depressed semicircle in moderate frequency region and a straight line in low-frequency regions, which are releure 3 d). The mesoporous SnO2 and SnO2/C electrodes were vant to a charge-transfer process and a Warburg-diffusion tested at discharge currents of 0.1, 0.2, 0.5, 1, 2, 5 and process, respectively. In high-frequency regions, the SnO2/C 10 A g 1 and then regularly returned to low current rates of
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1 and 0.1 A g 1, with 5 cycles for each step. The capacity of uncoated mesoporous SnO2 fades rapidly with increasing current and cannot recover to initial levels even at low charge/discharge currents after high-rate cycling. The capacity of uncoated SnO2 at 10 A g 1 (approximate to 12.7 C, based on the theoretic specific capacity of SnO2 790 mAh g 1) is only 100 mAh g 1, indicating a poor highrate performance. The mesoporous SnO2/C electrode exhibits high-rate capacity (510 mAh g 1 at a current of 10 A g 1) and excellent cycling property under high-rate testing. Furthermore, when the testing currents were regularly returned to low-current rates, such as 1 and 0.1 A g 1, the discharge capacities recovered to 725 and 930 mAh g 1, which were nearly the same as previous measurements. The overall electrochemical performance for this mesoporous core/shell structured SnO2/C electrode material is superior to the previous reports of SnO2/C materials.[7g–l, 9] In summary, ordered mesoporous SnO2/C composites consisting of SnO2 crystalline core and amorphous carbon shell were prepared by a facile vacuum-assisted impregnation method. The as-prepared SnO2/C composite presents high surface area, large pore volume and narrow pore-size distribution, which are beneficial to lithium-ion intercalation and electrolyte diffusion. The SnO2/C electrode exhibits a lower resistance and higher charge transfer dynamics compared to uncoated mesoporous SnO2, due to the improvement introduced by the carbon coating. Thus, the SnO2/C material achieves an excellent electrochemical performance as an anode material for lithium-ion batteries. The discharge capacity of mesoporous SnO2/C at 0.1 A g 1 after 100 cycles is 780 mAh g 1, which is more than double the theoretical capacity of commercial graphite material. The capacity at discharge current of 10 A g 1 is 510 mAh g 1, illustrating the promising potential of this mesoporous SnO2/C as a highpower source for next-generation EVs.
range from 208 to 808, by using Co radiation. Thermogravimetric analysis (TGA) was carried out by a SDT Q600 analyser, Raman spectroscopy of mesoporous SnO2/C was collected by using an HR Micro-Raman spectrometer. N2 adsorption–desorption isotherms of mesoporous SnO2/C were measured by using a Quadrasorb SI analyser at the testing temperature of 77 K. Brunauer–Emmett–Teller (BET) was used to calculate the surface area and Barret–Joyner–Halenda (BJH) was used to determine the pore-size distribution, respectively. The BET surface area was calculated by using points at a relative pressure of P/P0 = 0.05–0.27. The poresize distribution was derived from the adsorption branch by using the BJH method. The total pore volume was determined by the nitrogen amount adsorbed at a relative pressure (P/P0) of 0.99. The morphology of mesoporous SnO2/C was observed with a field emission scanning electron microscope (FESEM, Zeiss Supra 55VP). The crystalline microstructures were observed by using transmission electron microscopy (TEM) and high-resolution TEM analysis (JEOL 2100, accelerating voltage 200 kV). Cell assembly and electrochemical testing: The mesoporous SnO2 and SnO2/C electrodes were fabricated by mixing the active materials with acetylene black (AB) and a binder, poly(vinylidene fluoride) (PVDF), at weight ratio of 70:20:10. The mixture was dispersed in n-methyl pyrrolidone (NMP) solvent to form a slurry. The slurry was uniformly pasted on Cu foil with a blade. The electrodes were dried at 120 8C in a vacuum oven for 12 h and subsequently pressed under a pressure of 200 kg cm 2. CR2032-type coin cells were assembled in a glove-box for electrochemical characterisation. A non-aqueous solution of LiPF6 (1 m) in a 1:1:1 of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) was used as the electrolyte (LB315, GuoTaiHuaRong Co., Ltd.). Li metal disks were used as the counter electrodes for electrochemical testing. Cyclic voltammetry (CV) curves were collected by using an electrochemistry workstation (CHI660C) at scanning rate of 0.1 mV s 1 within a range of 0.05–3.0 V. The electrochemical impedance spectroscopy of mesoporous SnO2 and SnO2/C fresh cells were measured by a.c. impedance method with an applied frequency from 10 m to 100 k Hz at a voltage of 2.0 V, by using a CHI electrochemical station. The cells were galvanostatically charged and discharged at a current density of 0.1 A g 1 within the voltage range of 0.05–3 V. For the high-rate testing, the charge/discharge current gradually increased from 0.1 A g 1 to 0.2, 0.5, 1, 2, 5 and 10 A g 1, then decreased to 1 and 0.1 A g 1, step by step.
Acknowledgements Experimental Section Synthesis of materials: 2-Dimensional (2D) hexagonal mesoporous SiO2 template (SBA-15) was synthesised by following the previous report.[13a] The mesoporous SnO2/C replica from the 2D hexagonal SBA-15 was synthesised by a vacuum assisted impregnation route.[13b] Butyltin trichloride (2 mmol, C4H9SnCl3, Aldrich, FW = 282.18 g mol 1) was dissolved in ethanol (15 mL) to form a transparent solution. Then, dry SBA-15 (0.15 g) was added to the solution. After being stirred, overnight, the solvent was removed by evaporation at room temperature in a vacuum oven (< 10 3 bar). The tin precursor/SBA15 composite was grounded and sintered at 400 8C in argon to decompose the organic tin source. The impregnation/decomposition step was repeated twice and finally calcined at 450 8C for 5 h in Ar. The silica template was removed by 10 wt. % HF. The mesoporous SnO2/C product was obtained after being filtered and washed with deionised water and ethanol several times, and then dried at 50 8C. For comparison, mesoporous SnO2 was synthesised by a similar impregnation method by using SnCl4·5 H2O as a tin source. Characterisations: The small angle X-ray diffraction (SAXRD) pattern of mesoporous SnO2/C was collected with a Bruker D8 Advanced X-ray diffractometer within the scanning range from 0.58 to 58 by using CuKa radiation (l = 0.15406 nm). Wide angle X-ray diffraction patterns of mesoporous SnO2/C was obtained from a Siemens D5000 XRD device in 2q
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This work was financially supported by the Australian Research Council (ARC) through the Discovery Project programs (DP1095861, DP130104459). H.L. would like to thank the support from UTS Chancellors Postdoctoral Fellowship (CPDF). G.X.W. appreciates the support from ARC Future Fellow Project (FT1101100800).
Keywords: composites · energy conversion · lithium-ion batteries · mesoporous materials · tin oxide
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Received: August 29, 2013 Published online: November 4, 2013
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Ordered Mesoporous Core/Shell SnO2/C Nanocomposite as High-Capacity Anode Material for Lithium-Ion Batteries Hao Liu,[a] Sheng Chen,[b] Guoxiu Wang,*[a] and Shi Zhang Qiao*[b]
Batteries are widely considered as the best power sources for the next generation of electric vehicles (EVs) in order to relieve our reliance on non-renewable energy sources.[1] Lithium-ion batteries are considered as one of the most promising battery options for EVs, because of their highenergy density compared to conventional rechargeable batteries.[2] However, current lithium-ion batteries consisting of microsize electrode materials are not competent as highpower sources; this is attributed to the polarisation and pulverisation of microsize electrode materials operating at high current.[3] Thus, it is critical to develop new electrode materials with enhanced electrochemical performance for EVs. Tin-based oxide materials have high capacity, because a large number of lithium ions can be hosted per Sn unit to form LixSn (x 4.4) alloy.[4] To date, many nanostructured SnO2 materials, including nanorods, nanowires, nanotubes, nanosheets, nanoboxes and mesopurous SnO2, have been widely investigated as anode materials for lithium-ion batteries, because nanostructured SnO2 materials provide a short pathway for lithium-ion diffusion.[5] However, lithiation-induced volume expansion causes pulverisation of electrode materials, and hence degrades the performance and lifetime of SnO2 anode.[6] Therefore, many attempts have been made to prevent SnO2 from pulverisation, by coating SnO2 with oxides, Si and carbon.[7] In particular, carbon coating has been extensively developed because it is a cheap, abundant and low-toxic source to enhance the conductivity and stability of SnO2 anode materials.[7g–o] Recently, mesostructured materials have attracted great attention because of their unique structure and high performance for high-power energy storage.[8] Especially, mesoporous carbon-based composites have been widely investigated for high-power energy storage, by embedding active materials into pores of mesoporous carbon substrates.[9] The electrochemical property of active materials can be remarkably en[a] Dr. H. Liu, Prof. Dr. G. Wang School of Chemistry and Forensic Science University of Technology Sydney Broadway Sydney, NSW 2007 (Australia) E-mail: [email protected] [b] Dr. S. Chen, Prof. Dr. S. Z. Qiao School of Chemical Engineering The University of Adelaide, SA 5005 (Australia) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303400.
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hanced by confinement of mesoporous carbon matrix. Many mesoporous SnO2/C nanocomposites have been reported.[10] For example, Huang et al. reported a sonochemical synthesis of ordered SnO2/CMK3 composite with enhanced electrochemical performance.[10a] Lu et al. fabricated a tubular mesoporous carbon substrate (CMK5) and homogeneously dispersed SnO2 nanoparticles in the mesopore channels. The as-prepared nanocomposite exhibited high reversible capacity and fast charge/discharge kinetics at high rate.[10b] Lou et al. prepared carbon supported mesoporous SnO2 by a hydrothermal method, and achieved high capacity and good retention.[10c] However, the above-mentioned reports of mesoporous SnO2/C composites were all based on two-step synthesis, namely, production of SnO2 and carbon separately. Herein, we report a facile vacuum-assisted impregnation method to produce mesoporous SnO2/C nanocomposites with simultaneous formation of SnO2 and carbon, using mesoporous silica (SBA15) as a template and organic tin salt (C4H9SnCl3) as a tin source. This SnO2/C with high surface area and large pore volume can provide more space for electrolyte soakage and possess more sites for lithium-ion intercalation. During the high-temperature calcination, alkyl groups were carbonised and coated on the surface of crystal SnO2, to form a core/shell nanostructure. This carbon layer can enhance the conductivity of SnO2 and suppress the volume change, and hence improve its electrochemical property as anode material for lithium-ion batteries. Figure 1 a shows the small-angle X-ray diffraction (SAXRD) pattern of the highly ordered mesoporous SnO2/ C composite. The diffraction peaks at 0.98 can be indexed to the corresponding diffractions of (100) in the hexagonal P6mm space groups. The wide angle XRD pattern of mesoporous SnO2/C presented in the inset of Figure 1 a exhibits diffraction peaks between 208 and 808, which can be indexed to the standard tetragonal Cassiterite SnO2 structure (JCPDS No. 41-1455). No impurity diffraction peaks were observed, suggesting a complete transformation from organic tin precursor to crystalline tin oxide. The component of SnO2/C was confirmed by Raman microscopy (Figure 1 b). The peaks at 485 and 618 cm 1 correspond to the typical Eg and A1g vibrational modes of the Sn O bond.[11a] The bonds at 1368 and 1571 cm 1 are the typical D and G lines of carbon material.[11b] The ratio of SnO2 and carbon in composite was examined by thermogravimetric analysis (TGA; Figure 1 c). The composite lost its weight between 380 and 510 8C due to the combustion of carbon in air, which is con-
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strates 2D mesopores the same as shown in the high-resolution SEM image. The selected area electron diffraction (SAED) pattern (Figure 2 b, inset) reveals the polycrystalline structure of the mesoporous SnO2/C, which is consistent with the XRD result. Figure 2 c clearly illustrates the 2D mesoporous channels between carboncoated crystal SnO2 nanowirelike arrays. The schematic core/ shell structure is shown as the inset of Figure 2 c. The core of simulated SnO2 shows unit cells of typical rutile structure, wherein the tin atoms are sixcoordinate and the oxygen atoms three-coordinate. The high-resolution TEM image of SnO2 clearly demonstrates crysFigure 1. Characterisations of mesoporous SnO2/C. a) Small-angle XRD and wide-angle XRD (inset) patterns. talline SnO2 coated by carbon b) Raman microscopy. c) TGA and heat flow curves. d) Nitrogen sorption isotherm and corresponding pore layers (Figure 2 d). The intersize distribution (inset). planar distances of observed (110) and (101) planes are well matched with standard rutile SnO2 structure. The fast Fourisistent with an exothermal process as shown in the heat-flow curve. The carbon ratio is 13.8 %, calculated from the TGA er transformation (FFT) pattern reveals the orientations result. The porosity of the SnO2/C composite was investigatalong different directions (Figure 2 d, inset). Amorphous ed by the nitrogen sorption measurement (Figure 1 d). The nitrogen sorption isotherm of mesoporous SnO2/C demonstrates an IV type isotherm. The specific surface area is 181 m2 g 1 and pore volume is 0.22 cm3 g 1, respectively. The pore-size distribution of the mesoporous SnO2/C calculated by the Barret–Joyner–Halenda (BJH) method is presented in Figure 1 d (inset). The relatively narrow pore-size distribution at 2.9 nm is attributed to 2-dimensional (2D) hexagonal mesoporous channels in SnO2/C, replicated from the walls of silica template. The above-mentioned results clearly demonstrate that the mesoporous SnO2/C material has been successfully replicated by this vacuum-assisted method from 2D hexagonal SBA-15 silica template (SAXRD and nitrogen sorption results of SBA-15 are shown in Figures S1 and S2 in the Supporting Information). Figure 2 shows scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the mesoporous SnO2/C material. The SEM image of mesoporous SnO2/C demonstrates a large-scale rod-like SnO2/C with particle sizes of 300 nm–1 micron (Figure 2 a). The morphology of mesoporous SnO2/C is not identical to the silica template because of volume shrinkage of crystals during the hightemperature treatment.[12] The inset of Figure 2 a is a highFigure 2. a) Low-magnification SEM image of mesoporous SnO2/C particles and high-resolution SEM image of ordered wire array structure resolution SEM of the composite, which clearly shows typi(inset). b) Low-magnification TEM view of and corresponding selected cal 2D wire-like arrays, with mesopores around 3 nm (SEM area electron diffraction (SAED) pattern. c) High-resolution TEM image image of SBA-15 template is shown in Figure S3 in the Supof core/shell structured SnO2/C and its simulated scheme shown in the porting Information). Figure 2 b presents a low-magnificainset. d) Crystal lattice fringe of core/shell SnO2/C and its corresponding FFT pattern in the inset. tion TEM image of a single SnO2/C particle, which demon-
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electrode shows lower values across the real resistance (Rre) axis, demonstrating a lower overall resistance of SnO2/C electrode than uncoated SnO2 electrode, which can be attributed to the enhancement of conductivity caused by the carbon coating. Moreover, the SnO2/C electrode exhibits a lower charge-transfer resistance, as shown in moderate frequency regions, which can lead to faster lithium-ion diffusion in solid state. Galvanostatical charge/discharge tests of mesoporous SnO2 and SnO2/C were investigated at a current density of 0.1 A g 1 within the voltage range of 0.05 and 3 V for 100 cycles. The cycling performance and the Coulombic effiFigure 3. a) CV curves of mesoporous SnO2/C for the first 3 cycles, with scanning rate of 0.1 mV s 1. b) Nyquist ciencies of mesoporous SnO2 plots of mesoporous SnO2 and SnO2/C. c) Lithium storage performance of mesoporous SnO2 and SnO2/C at and SnO2/C electrodes are the galvanostatic current of 0.1 A g 1. d) Multirate tests of mesoporous SnO2 and SnO2/C, in order of 0.1, 0.2, 0.5, 1, 2, 5, 10 A g 1, and then return to low currents of 1 and 0.1 A g 1, with 5 cycles for each step. shown in Figure 3 c. The initial discharge capacities of the mesoporous SnO2 and SnO2/C electrodes are 1654 and 1696 mAh g 1, respectively. The high inicarbon layers of around 1 nm are denoted in Figure 2 d. The carbon shell will be beneficial for enhancing electronic contial discharge capacity (double the theoretical capacity) can ductivity and suppressing the volume expansion during be attributed to the two steps of lithium-ion intercalations: charge/discharge. reduction of Sn cation to Sn0 and subsequent formation of Figure 3 a shows cyclic voltammetry (CV) curves of mesoLixSn alloy, accompanied by the decomposition of non-aqueporous SnO2/C for the first 3 cycles, in the scanning region ous electrolyte. In subsequent cycles, the discharge capacity of mesoporous SnO2 electrode dramatically decreases and between 0.05 and 3 V. In the initial CV cycle, the reduction only delivers a capacity of 411 mAh g 1 after 100 cycles. In peaks at 0.8 V are attributed to the lithium insertion to SnO2 contrast, the discharge capacity of mesoporous SnO2/C after to form Sn and Li2O, accompanied by the decomposition of non-aqueous electrolyte to form a solid electrolyte inter100 cycles remained as high as 780 mAh g 1 (more than phase (SEI) layer. Lithium ions subsequently insert to Sn double commercial graphite 372 mAh g 1), indicating and carbon to form LixSn and LixC6 compounds, down to a great improvement of electrochemical performance compared to the uncoated mesoporous SnO2 (discharge curves the cut-off voltage of 0.05 V. During the anodic scanning, there are three oxidation peaks presented at 0.56, 1.25 and of mesoporous SnO2 and SnO2/C for the 1st, 2nd and 100th 1.80 V, corresponding to the oxidation stages of lithium ions cycles are available in Figure S9 in the Supporting Informaextracted from LixSn alloy, consistent with the results prevition). Moreover, the mesoporous SnO2/C electrode exhibits ously described.[9b] The CV curves of the second and third higher initial Coulombic efficiency (65.1 %) than mesoporous SnO2 (63.8 %). In the following cycles, the efficiencies cycles are identical, indicating an excellent reversibility of the mesoporous SnO2/C electrode. In contrast, CV curves of of mesoporous SnO2/C electrodes gradually increase and uncoated mesoporous SnO2 show a distinguished decrease stay at a stable level after 5 cycles, at approximately 97 to 98 %, which is higher than the uncoated mesosporous SnO2 in the intensity of the redox peaks, as well as their integrated area (Figure S8 in the Supporting Information), demon(around 96 %). The improved long-term performance can be strating a fast capacity fading and worse electrochemical attributed to the enhanced stability of the SnO2/C composite performance than the SnO2/C. The Nyquist plots of mesopopromoted by the carbon-coating layer. Multiple-current galvanostatical tests were carried out to investigate the highrous SnO2 and SnO2/C are shown in Figure 3 b. Both profiles rate performance of mesoporous SnO2 and SnO2/C (Figpresent a depressed semicircle in moderate frequency region and a straight line in low-frequency regions, which are releure 3 d). The mesoporous SnO2 and SnO2/C electrodes were vant to a charge-transfer process and a Warburg-diffusion tested at discharge currents of 0.1, 0.2, 0.5, 1, 2, 5 and process, respectively. In high-frequency regions, the SnO2/C 10 A g 1 and then regularly returned to low current rates of
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1 and 0.1 A g 1, with 5 cycles for each step. The capacity of uncoated mesoporous SnO2 fades rapidly with increasing current and cannot recover to initial levels even at low charge/discharge currents after high-rate cycling. The capacity of uncoated SnO2 at 10 A g 1 (approximate to 12.7 C, based on the theoretic specific capacity of SnO2 790 mAh g 1) is only 100 mAh g 1, indicating a poor highrate performance. The mesoporous SnO2/C electrode exhibits high-rate capacity (510 mAh g 1 at a current of 10 A g 1) and excellent cycling property under high-rate testing. Furthermore, when the testing currents were regularly returned to low-current rates, such as 1 and 0.1 A g 1, the discharge capacities recovered to 725 and 930 mAh g 1, which were nearly the same as previous measurements. The overall electrochemical performance for this mesoporous core/shell structured SnO2/C electrode material is superior to the previous reports of SnO2/C materials.[7g–l, 9] In summary, ordered mesoporous SnO2/C composites consisting of SnO2 crystalline core and amorphous carbon shell were prepared by a facile vacuum-assisted impregnation method. The as-prepared SnO2/C composite presents high surface area, large pore volume and narrow pore-size distribution, which are beneficial to lithium-ion intercalation and electrolyte diffusion. The SnO2/C electrode exhibits a lower resistance and higher charge transfer dynamics compared to uncoated mesoporous SnO2, due to the improvement introduced by the carbon coating. Thus, the SnO2/C material achieves an excellent electrochemical performance as an anode material for lithium-ion batteries. The discharge capacity of mesoporous SnO2/C at 0.1 A g 1 after 100 cycles is 780 mAh g 1, which is more than double the theoretical capacity of commercial graphite material. The capacity at discharge current of 10 A g 1 is 510 mAh g 1, illustrating the promising potential of this mesoporous SnO2/C as a highpower source for next-generation EVs.
range from 208 to 808, by using Co radiation. Thermogravimetric analysis (TGA) was carried out by a SDT Q600 analyser, Raman spectroscopy of mesoporous SnO2/C was collected by using an HR Micro-Raman spectrometer. N2 adsorption–desorption isotherms of mesoporous SnO2/C were measured by using a Quadrasorb SI analyser at the testing temperature of 77 K. Brunauer–Emmett–Teller (BET) was used to calculate the surface area and Barret–Joyner–Halenda (BJH) was used to determine the pore-size distribution, respectively. The BET surface area was calculated by using points at a relative pressure of P/P0 = 0.05–0.27. The poresize distribution was derived from the adsorption branch by using the BJH method. The total pore volume was determined by the nitrogen amount adsorbed at a relative pressure (P/P0) of 0.99. The morphology of mesoporous SnO2/C was observed with a field emission scanning electron microscope (FESEM, Zeiss Supra 55VP). The crystalline microstructures were observed by using transmission electron microscopy (TEM) and high-resolution TEM analysis (JEOL 2100, accelerating voltage 200 kV). Cell assembly and electrochemical testing: The mesoporous SnO2 and SnO2/C electrodes were fabricated by mixing the active materials with acetylene black (AB) and a binder, poly(vinylidene fluoride) (PVDF), at weight ratio of 70:20:10. The mixture was dispersed in n-methyl pyrrolidone (NMP) solvent to form a slurry. The slurry was uniformly pasted on Cu foil with a blade. The electrodes were dried at 120 8C in a vacuum oven for 12 h and subsequently pressed under a pressure of 200 kg cm 2. CR2032-type coin cells were assembled in a glove-box for electrochemical characterisation. A non-aqueous solution of LiPF6 (1 m) in a 1:1:1 of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) was used as the electrolyte (LB315, GuoTaiHuaRong Co., Ltd.). Li metal disks were used as the counter electrodes for electrochemical testing. Cyclic voltammetry (CV) curves were collected by using an electrochemistry workstation (CHI660C) at scanning rate of 0.1 mV s 1 within a range of 0.05–3.0 V. The electrochemical impedance spectroscopy of mesoporous SnO2 and SnO2/C fresh cells were measured by a.c. impedance method with an applied frequency from 10 m to 100 k Hz at a voltage of 2.0 V, by using a CHI electrochemical station. The cells were galvanostatically charged and discharged at a current density of 0.1 A g 1 within the voltage range of 0.05–3 V. For the high-rate testing, the charge/discharge current gradually increased from 0.1 A g 1 to 0.2, 0.5, 1, 2, 5 and 10 A g 1, then decreased to 1 and 0.1 A g 1, step by step.
Acknowledgements Experimental Section Synthesis of materials: 2-Dimensional (2D) hexagonal mesoporous SiO2 template (SBA-15) was synthesised by following the previous report.[13a] The mesoporous SnO2/C replica from the 2D hexagonal SBA-15 was synthesised by a vacuum assisted impregnation route.[13b] Butyltin trichloride (2 mmol, C4H9SnCl3, Aldrich, FW = 282.18 g mol 1) was dissolved in ethanol (15 mL) to form a transparent solution. Then, dry SBA-15 (0.15 g) was added to the solution. After being stirred, overnight, the solvent was removed by evaporation at room temperature in a vacuum oven (< 10 3 bar). The tin precursor/SBA15 composite was grounded and sintered at 400 8C in argon to decompose the organic tin source. The impregnation/decomposition step was repeated twice and finally calcined at 450 8C for 5 h in Ar. The silica template was removed by 10 wt. % HF. The mesoporous SnO2/C product was obtained after being filtered and washed with deionised water and ethanol several times, and then dried at 50 8C. For comparison, mesoporous SnO2 was synthesised by a similar impregnation method by using SnCl4·5 H2O as a tin source. Characterisations: The small angle X-ray diffraction (SAXRD) pattern of mesoporous SnO2/C was collected with a Bruker D8 Advanced X-ray diffractometer within the scanning range from 0.58 to 58 by using CuKa radiation (l = 0.15406 nm). Wide angle X-ray diffraction patterns of mesoporous SnO2/C was obtained from a Siemens D5000 XRD device in 2q
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This work was financially supported by the Australian Research Council (ARC) through the Discovery Project programs (DP1095861, DP130104459). H.L. would like to thank the support from UTS Chancellors Postdoctoral Fellowship (CPDF). G.X.W. appreciates the support from ARC Future Fellow Project (FT1101100800).
Keywords: composites · energy conversion · lithium-ion batteries · mesoporous materials · tin oxide
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Received: August 29, 2013 Published online: November 4, 2013
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