DOI: 10.1002/chem.201404077
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& Energy-Storage Materials
Electrochemical Properties of Tin Oxide Flake/Reduced Graphene Oxide/Carbon Composite Powders as Anode Materials for Lithium-Ion Batteries Su Min Lee,[a] Seung Ho Choi,[a] and Yun Chan Kang*[b]
Abstract: Hierarchically structured tin oxide/reduced graphene oxide (RGO)/carbon composite powders are prepared through a one-pot spray pyrolysis process. SnO nanoflakes of several hundred nanometers in diameter and a few nanometers in thickness are uniformly distributed over the micrometer-sized spherical powder particles. The initial discharge and charge capacities of the tin oxide/RGO/carbon composite powders at a current density of 1000 mA g 1 are 1543 and 1060 mA h g 1, respectively. The discharge capacity of
Introduction Tin oxide materials including SnO2 and SnO have been studied extensively as anode materials for lithium ion batteries (LIBs), and have attracted considerable attention because of their high theoretical capacities.[1–17] A variety of nanostructured materials prepared by solution- and gas-phase reaction methods have been studied to improve the electrochemical properties of tin oxide.[18–21] However, problems in the handling and printing of nanostructured materials, including nanopowders, nanorods, nanoplates, and so on, make it difficult to achieve electrodes with high tap densities. Therefore, carbon-related materials incorporating tin oxide nanocrystals have been developed as efficient anode materials for LIBs.[22–26] Tin oxide nanocrystals embedded in the carbon matrix can accommodate the large volume changes that occur during the charge/discharge process and provide a stable cycling performance.[24–27] A carbon matrix with high electronic conductivity can also improve the electrochemical properties of the nanostructured materials. The embedding of tin oxide nanocrystals into micron-sized carbon microspheres can therefore solve the problem of low tap densities for nanostructured materials as well as providing stable electrochemical properties.[28] [a] S. M. Lee, S. H. Choi Department of Chemical Engineering, Konkuk University 1 Hwayang-Dong, Gwangjin-Gu, Seoul 143–701 (Korea) [b] Prof. Y. C. Kang Department of Materials Science and Engineering Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713 (Korea) Fax: (+ 82) 2-928-3584 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404077. Chem. Eur. J. 2014, 20, 15203 – 15207
the tin oxide/RGO/carbon composite powders after 175 cycles is 844 mA h g 1, and the capacity retention measured from the second cycle is 80 %. The transformation during cycling of SnO nanoflakes, uniformly dispersed in the tin oxide/RGO/carbon composite powder, into ultrafine nanocrystals results in hollow nanovoids that act as buffers for the large volume changes that occur during cycling, thereby improving the cycling and rate performances of the tin oxide/RGO/carbon composite powders.
Graphene materials incorporating nanocrystals of active material show superior electrochemical properties.[29–34] In particular, spray pyrolysis was applied successfully in the preparation of reduced graphene oxide (RGO) microspheres uniformly embedded with metal oxide nanocrystals.[35, 36] In this study, hierarchically structured tin oxide/RGO/carbon composite materials with controlled morphologies were prepared by using a onepot spray pyrolysis process. Sucrose, dissolved into the spray solution as the carbon source material, affected the morphologies as well as the crystal structures of the tin oxide/RGO/ carbon composite powders. The electrochemical properties of the hierarchical structured tin oxide/RGO/carbon composite materials were compared with those of tin oxide/RGO, tin oxide/carbon, and bare tin oxide powders prepared through the same process but starting from different types of spray solution.
Results and Discussion The morphologies of the tin oxide/RGO/carbon composite powders prepared by spray pyrolysis are shown in Figure 1. The composite powders have a hierarchical structure consisting of nanoflakes of several hundred nanometers in diameter and a few nanometers in thickness, which are distributed uniformly over spherical, micrometer-sized powder particles. Ultrafine, nanometer-scale powders are also uniformly distributed over the composite powder, as shown by the arrows in Figure 1 b. The high-resolution TEM images shown in Figure 1 d reveal clear lattice fringes separated by 0.26 and 0.33 nm, which correspond to the (101) and (110) crystal planes of SnO and SnO2, respectively. The nanoflakes and nanopowders seen in the TEM images are SnO and SnO2, respectively. The elemental mapping images shown in Figure 1 e reveal uniform distri-
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Figure 2. TEM and dot-mapping images of the tin oxide/RGO composite powders prepared by spray pyrolysis.
Figure 1. TEM and dot-mapping images of the tin oxide/RGO/carbon composite powders prepared by spray pyrolysis.
butions of Sn and C over the composite powders, the latter originating from the RGO and carbon material formed by the carbonization of sucrose. The morphologies of the tin oxide/RGO powders prepared from the spray solution without sucrose are shown in Figure 2, in which full spherical particles are observed. The high-resolution TEM image shown in Figure 2 c shows ultrafine nanocrystals dispersed uniformly on the RGO layer. The inset image reveals clear lattice fringes separated by 0.33 nm, corresponding to (110) tetragonal SnO2. The elemental mapping images shown in Figure 2 d highlight the uniform distributions of Sn and C, the latter coming from the RGO. The morphologies of the bare tin oxide powders prepared from the spray solution without the carbon source material are shown in Figure S1 of the Supporting Information; the powder particles are perfectly spherical in shape, with a smooth surface and filled morphology. The high-resolution TEM image in Figure S1 d shows SnO2 nanocrystals of several nanometers in size. The tin oxide/ carbon composite powder particles shown in Figure S2 are also spherical. XRD patterns of the samples prepared from the various types of spray solution are shown in Figure 3 a. The XRD patterns from both the bare SnO2 and tin oxide/RGO powders prepared from the spray solutions without sucrose exhibit diffraction peaks corresponding to pure tetragonal SnO2 (JCPDS no. 41–1445), irrespective of the presence of RGO. However, the XRD patterns from the tin oxide/RGO/carbon and tin oxide/carbon composite powders prepared from the spray solChem. Eur. J. 2014, 20, 15203 – 15207
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utions with sucrose show signals corresponding to mixed phases of SnO (JCPDS no. 06-0395) and SnO2. The reducing atmosphere around the powder particles caused by the carbon material formed by the carbonization of sucrose produces oxygen-deficient SnO material. However, the short residence time (8 s) of the powders inside the hot wall reactor (maintained at 800 8C) results in tin oxide/RGO/carbon and tin oxide/ carbon composite powders with a mixed crystal structure of SnO and SnO2 phases. The mole ratio of SnO to SnO2 in the tin oxide/RGO/carbon composite powder evaluated from the XPS spectrum shown in Figure S3 (Supporting Information) was 21:79. Figure 3 b shows the Barrett–Joyner–Halenda (BJH) poresize distributions of the four samples. The bare SnO2 powders have a dense structure without pores, whereas the spray solutions in which sucrose is dissolved result in mesopores of less than 3 nm in diameter within the tin oxide/carbon and tin oxide/RGO/carbon composite powders. However, the tin oxide/RGO composite powders contain uniform mesopores with diameters of around 3.5 nm. The BET specific surface areas of the tin oxide/RGO/carbon, tin oxide/RGO, tin oxide/ carbon, and bare tin oxide powders were 103, 44, 37, and 5 m2 g 1, respectively. Figure 3 c shows the thermogravimetric (TG) curves for the tin oxide/RGO/carbon and tin oxide/RGO composite powders. Both show a weight decrease below 500 8C, which can be attributed to the decomposition of carbon and RGO. From the TG analysis, the RGO content in the tin oxide/RGO composite powders was estimated to be 4 wt %. The weight decrease in the tin oxide/RGO/carbon composite powders caused by the decomposition of carbon and RGO was compensated somewhat by the weight increase caused by the oxidation of SnO to SnO2. Therefore, the minimum carbon and RGO content in the tin oxide/RGO/carbon composite powders was estimated to be 17 wt %. Figure 3 d shows
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Full Paper RGO, tin oxide/carbon, and bare tin oxide powders were 1543, 1558, 1279, and 1484 mA h g 1, respectively. Figure 4 c shows the cycling performances of the four samples at a constant current density of 1000 mA g 1. The discharge capacities of the tin oxide/RGO/carbon, tin oxide/ RGO, tin oxide/carbon, and bare tin oxide powders after 175 cycles were 844, 331, 175, and 91 mA h g 1, respectively, and their capacity retentions measured from the second cycles were 80, 28, 19, and 9 %. All impedance measurements were performed at room temperature after 50 cycles at a current density of 1000 mA g 1. The resulting Nyquist plots (Figure 4 d) show a semicircle in the medium frequency region, assigned to the charge-transfer resistance of the Figure 3. a) XRD patterns, b) pore-size distributions, c) TG curves, and d) Raman spectra of the powders prepared electrodes, and a line inclined to from the various types of spray solutions. the real axis at low frequencies, which corresponds to lithium diffusion within the electrodes.[39–41] The tin oxide/RGO/carbon Raman spectra of the tin oxide/RGO/carbon composite and bare SnO2 powders. Two characteristic D and G graphene composite powders have the lowest charge-transfer resistance after cycling, as shown in Figure 4 d. The relationship between bands are observed at 1323 and 1588 cm 1, respectively, for the real part of the impedance spectra (Zre) and w 1/2 (w is the the composite powders, which are not observed in the spectrum of the bare SnO2 powders. In the preparation of tin angular frequency, w = 2pf) in the low-frequency region, as shown in Figure 4 e, reveals a higher lithium-ion diffusion rate oxide/RGO/carbon composite powders by one-pot spray pyrolfor the tin oxide/RGO/carbon composite powders than for the ysis, RGO is formed by the thermal reduction of graphene other samples.[40, 41] These results are attributable to the strucoxide sheets.[35, 36] The cyclic voltammograms (CVs) obtained for the tin oxide/ tural stability of the tin oxide/RGO/carbon composite powders RGO/carbon composite powders for the first ten cycles at during cycling, which lowers the charge-transfer resistance and a scan rate of 0.07 mV s 1 are shown in Figure 4 a. The CV thereby improves the lithium-ion diffusion rate. Tin oxide/RGO/ curve for the first discharging process shows two reduction carbon composite powders with high structural stability have peaks, at 0.16 and 0.87 V. The reduction peak at 0.87 V is assobetter rate performances than bare tin oxide powders, as ciated with the formation of metallic Sn nanograins and amorshown in Figure 4 f. TEM images of the tin oxide/RGO/carbon phous Li2O through the reduction of SnO2 and SnO.[18] The recomposite and bare tin oxide powders after 100 cycles are duction peak at 0.16 V is attributed to the formation of LixSn shown in Figure 5. After cycling, the spherical morphology of alloys.[37] The strong broad peak at approximately 0.52 V in the the particles is lost for the bare tin oxide powders. However, first charging process is attributed to Li dealloying from the tin oxide/RGO/carbon composites retained their spherical LixSn.[18] The reduction peak at 0.87 V disappeared after the morphology after cycling. Hollow nanovoids distributed uniformly within the composite powder are indicated by the first cycle, indicating that only the reversible conversion of Sn arrows in the TEM images, and are the result of the transforto LixSn was allowed in subsequent cycles.[37] The good overlap mation during cycling of SnO nanoflakes, uniformly dispersed with the CV curves from the second cycle reveals the reversibilin the tin oxide/RGO/carbon composite powder, into ultrafine ity of the process within the tin oxide/RGO/carbon composite nanocrystals. The high-resolution TEM images shown in Figpowders. The initial charge and discharge curves at a current ure 5 b and 5 d reveal the amorphous-like structure of the density of 1000 mA g 1, shown in Figure 4 b, have similar powder particles after cycling. The hollow nanovoids act as shapes for the four samples. The potential plateaus in the inibuffers for the large volume changes that occur during cycling, tial discharge curves at approximately 1.0 V are related to the such that the tin oxide/RGO/carbon composite powders have reduction of SnO and SnO2 to metallic Sn nanograins and superior cycling and rate performances. amorphous Li2O, as described for the CV results.[26] The initial discharge capacities of the tin oxide/RGO/carbon, tin oxide/ Chem. Eur. J. 2014, 20, 15203 – 15207
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Full Paper Experimental Section Synthesis of powders Graphene oxide (GO) was synthesized from graphite flakes following a modified Hummers The as-obtained method.[35, 38] graphite oxide was redispersed in distilled water and exfoliated ultrasonically to generate graphene oxide sheets. The graphene oxide sheets thus obtained were used to prepare the spray solution by dissolving tin oxalate (SnC2O4 ; 5.3 g) in exfoliated graphene oxide dispersion (400 mL; 1 mg mL 1). The tin oxide/RGO/carbon composite powders were prepared directly by spray pyrolysis from the colloidal spray solution containing GO, sucrose, and tin oxalate. Tin oxide/ graphene composite powders were prepared from the above colloidal spray solution without sucrose. Tin oxide/carbon and bare tin oxide powders were prepared from the spray solutions with and without sucrose, respectively. The ultrasonic spray pyrolysis system used in this study was described in our previous articles.[43, 44] In the experiment, a quartz reactor of 1200 mm in length and 50 mm in diameter was maintained at 800 8C. The nitrogen flow rate (carrier gas) during spray pyrolysis was maintained at 10 L min 1. Figure 4. Electrochemical properties of the four samples: a) CV curves of the tin oxide/RGO/carbon, b) initial charge and discharge curves, c) cycling performances, d) Nyquist plots after 50 cycles, e) relationships between the real part of the impedance (Zre) and w 1/2, and f) rate performances.
Conclusion The electrochemical properties of tin oxide/RGO/carbon composite powders as anode materials for LIBs were compared to those of tin oxide/RGO, tin oxide/carbon, and bare tin oxide powders prepared through the same process. Sucrose dissolved into the spray solution as the carbon source material enabled the formation of hierarchically structured tin oxide/ RGO/carbon composite powders. The reducing atmosphere around the powder particles caused by carbon material formed by the carbonization of sucrose produced SnO nanoflakes, which were dispersed uniformly throughout the tin oxide/RGO/carbon composite powder. The tin oxide/RGO/ carbon composite powders, with a mixed crystal structure of SnO and SnO2, had superior electrochemical properties to those of tin oxide/RGO, tin oxide/carbon, and bare tin oxide powders. Hollow nanovoids formed during cycling by the transformation of SnO nanoflakes into ultrafine nanocrystals improved the electrochemical properties of the tin oxide/RGO/ carbon composite powders. Chem. Eur. J. 2014, 20, 15203 – 15207
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Characterization
The crystal structures of the prepared powders were investigated by X-ray diffraction (XRD, X’pert PRO MPD) using CuKa radiation (l = 1.5418 ) at the Korea Basic Science Institute (Daegu). The morphologies of the prepared composites were observed by scanning electron microscopy (SEM, JEOL JSM-6060) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F) operating at a voltage of 200 kV. The specific surface areas of the powders were calculated from nitrogen adsorption measurements (TriStar 3000) using the Brunauer–Emmett–Teller (BET) method. Raman spectroscopic measurements were performed on a Nanofinder 30 spectrometer (Tokyo Instruments) using a He-Ne laser with a wavelength of 633 nm.
Electrochemical measurements The capacity and cycling properties of the prepared powders were determined by constructing a 2032-type coin cell. The electrode was prepared from a mixture containing 70 wt % active material, 20 wt % Super P, and 10 wt % sodium carboxymethyl cellulose (CMC) binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was composed of 1 m LiPF6 in a mixture of ethylene
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Figure 5. Morphologies of a,b) bare tin oxide and c,d) tin oxide/RGO/carbon composite after 100 cycles.
carbonate/dimethyl carbonate (EC/DMC; 1:1 by volume) containing 2 % vinylene carbonate. The charge/discharge characteristics of the samples were determined by cycling in the voltage range 0.001– 3 V. Cyclic voltammetry measurements were performed at a scan rate of 0.07 mV s 1.
Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A02046367). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (201320200000420). Keywords: energy conversion · energy-storage materials · graphene · nanostructures · synthesis design [1] L. Zhang, H. Wu, B. Liu, X. W. Lou, Energy Environ. Sci. 2014, 7, 1013 – 1017. [2] L. Zhang, G. Zhang, H. B. Wu, L. Yu, X. W. Lou, Adv. Mater. 2013, 25, 2589 – 2593. [3] J. S. Chen, X. W. Lou, Small 2013, 9, 1877 – 1893. [4] Z. H. Wen, Q. Wang, Q. Zhang, J. H. Li, Adv. Funct. Mater. 2007, 17, 2772 – 2778. [5] X. W. Lou, C. M. Li, L. A. Archer, Adv. Mater. 2009, 21, 2536 – 2539. [6] C. Wang, Y. Zhou, M. Y. Ge, X. B. Xu, Z. L. Zhang, J. Z. Jiang, J. Am. Chem. Soc. 2010, 132, 46 – 47. [7] Y. Wang, H. C. Zeng, J. Y. Lee, Adv. Mater. 2006, 18, 645 – 649.
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Received: June 23, 2014 Published online on September 29, 2014
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Electrochemical Properties of Tin Oxide Flake/Reduced Graphene Oxide/Carbon Composite Powders as Anode Materials for Lithium-Ion Batteries Su Min Lee,[a] Seung Ho Choi,[a] and Yun Chan Kang*[b]
Abstract: Hierarchically structured tin oxide/reduced graphene oxide (RGO)/carbon composite powders are prepared through a one-pot spray pyrolysis process. SnO nanoflakes of several hundred nanometers in diameter and a few nanometers in thickness are uniformly distributed over the micrometer-sized spherical powder particles. The initial discharge and charge capacities of the tin oxide/RGO/carbon composite powders at a current density of 1000 mA g 1 are 1543 and 1060 mA h g 1, respectively. The discharge capacity of
Introduction Tin oxide materials including SnO2 and SnO have been studied extensively as anode materials for lithium ion batteries (LIBs), and have attracted considerable attention because of their high theoretical capacities.[1–17] A variety of nanostructured materials prepared by solution- and gas-phase reaction methods have been studied to improve the electrochemical properties of tin oxide.[18–21] However, problems in the handling and printing of nanostructured materials, including nanopowders, nanorods, nanoplates, and so on, make it difficult to achieve electrodes with high tap densities. Therefore, carbon-related materials incorporating tin oxide nanocrystals have been developed as efficient anode materials for LIBs.[22–26] Tin oxide nanocrystals embedded in the carbon matrix can accommodate the large volume changes that occur during the charge/discharge process and provide a stable cycling performance.[24–27] A carbon matrix with high electronic conductivity can also improve the electrochemical properties of the nanostructured materials. The embedding of tin oxide nanocrystals into micron-sized carbon microspheres can therefore solve the problem of low tap densities for nanostructured materials as well as providing stable electrochemical properties.[28] [a] S. M. Lee, S. H. Choi Department of Chemical Engineering, Konkuk University 1 Hwayang-Dong, Gwangjin-Gu, Seoul 143–701 (Korea) [b] Prof. Y. C. Kang Department of Materials Science and Engineering Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713 (Korea) Fax: (+ 82) 2-928-3584 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404077. Chem. Eur. J. 2014, 20, 15203 – 15207
the tin oxide/RGO/carbon composite powders after 175 cycles is 844 mA h g 1, and the capacity retention measured from the second cycle is 80 %. The transformation during cycling of SnO nanoflakes, uniformly dispersed in the tin oxide/RGO/carbon composite powder, into ultrafine nanocrystals results in hollow nanovoids that act as buffers for the large volume changes that occur during cycling, thereby improving the cycling and rate performances of the tin oxide/RGO/carbon composite powders.
Graphene materials incorporating nanocrystals of active material show superior electrochemical properties.[29–34] In particular, spray pyrolysis was applied successfully in the preparation of reduced graphene oxide (RGO) microspheres uniformly embedded with metal oxide nanocrystals.[35, 36] In this study, hierarchically structured tin oxide/RGO/carbon composite materials with controlled morphologies were prepared by using a onepot spray pyrolysis process. Sucrose, dissolved into the spray solution as the carbon source material, affected the morphologies as well as the crystal structures of the tin oxide/RGO/ carbon composite powders. The electrochemical properties of the hierarchical structured tin oxide/RGO/carbon composite materials were compared with those of tin oxide/RGO, tin oxide/carbon, and bare tin oxide powders prepared through the same process but starting from different types of spray solution.
Results and Discussion The morphologies of the tin oxide/RGO/carbon composite powders prepared by spray pyrolysis are shown in Figure 1. The composite powders have a hierarchical structure consisting of nanoflakes of several hundred nanometers in diameter and a few nanometers in thickness, which are distributed uniformly over spherical, micrometer-sized powder particles. Ultrafine, nanometer-scale powders are also uniformly distributed over the composite powder, as shown by the arrows in Figure 1 b. The high-resolution TEM images shown in Figure 1 d reveal clear lattice fringes separated by 0.26 and 0.33 nm, which correspond to the (101) and (110) crystal planes of SnO and SnO2, respectively. The nanoflakes and nanopowders seen in the TEM images are SnO and SnO2, respectively. The elemental mapping images shown in Figure 1 e reveal uniform distri-
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Figure 2. TEM and dot-mapping images of the tin oxide/RGO composite powders prepared by spray pyrolysis.
Figure 1. TEM and dot-mapping images of the tin oxide/RGO/carbon composite powders prepared by spray pyrolysis.
butions of Sn and C over the composite powders, the latter originating from the RGO and carbon material formed by the carbonization of sucrose. The morphologies of the tin oxide/RGO powders prepared from the spray solution without sucrose are shown in Figure 2, in which full spherical particles are observed. The high-resolution TEM image shown in Figure 2 c shows ultrafine nanocrystals dispersed uniformly on the RGO layer. The inset image reveals clear lattice fringes separated by 0.33 nm, corresponding to (110) tetragonal SnO2. The elemental mapping images shown in Figure 2 d highlight the uniform distributions of Sn and C, the latter coming from the RGO. The morphologies of the bare tin oxide powders prepared from the spray solution without the carbon source material are shown in Figure S1 of the Supporting Information; the powder particles are perfectly spherical in shape, with a smooth surface and filled morphology. The high-resolution TEM image in Figure S1 d shows SnO2 nanocrystals of several nanometers in size. The tin oxide/ carbon composite powder particles shown in Figure S2 are also spherical. XRD patterns of the samples prepared from the various types of spray solution are shown in Figure 3 a. The XRD patterns from both the bare SnO2 and tin oxide/RGO powders prepared from the spray solutions without sucrose exhibit diffraction peaks corresponding to pure tetragonal SnO2 (JCPDS no. 41–1445), irrespective of the presence of RGO. However, the XRD patterns from the tin oxide/RGO/carbon and tin oxide/carbon composite powders prepared from the spray solChem. Eur. J. 2014, 20, 15203 – 15207
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utions with sucrose show signals corresponding to mixed phases of SnO (JCPDS no. 06-0395) and SnO2. The reducing atmosphere around the powder particles caused by the carbon material formed by the carbonization of sucrose produces oxygen-deficient SnO material. However, the short residence time (8 s) of the powders inside the hot wall reactor (maintained at 800 8C) results in tin oxide/RGO/carbon and tin oxide/ carbon composite powders with a mixed crystal structure of SnO and SnO2 phases. The mole ratio of SnO to SnO2 in the tin oxide/RGO/carbon composite powder evaluated from the XPS spectrum shown in Figure S3 (Supporting Information) was 21:79. Figure 3 b shows the Barrett–Joyner–Halenda (BJH) poresize distributions of the four samples. The bare SnO2 powders have a dense structure without pores, whereas the spray solutions in which sucrose is dissolved result in mesopores of less than 3 nm in diameter within the tin oxide/carbon and tin oxide/RGO/carbon composite powders. However, the tin oxide/RGO composite powders contain uniform mesopores with diameters of around 3.5 nm. The BET specific surface areas of the tin oxide/RGO/carbon, tin oxide/RGO, tin oxide/ carbon, and bare tin oxide powders were 103, 44, 37, and 5 m2 g 1, respectively. Figure 3 c shows the thermogravimetric (TG) curves for the tin oxide/RGO/carbon and tin oxide/RGO composite powders. Both show a weight decrease below 500 8C, which can be attributed to the decomposition of carbon and RGO. From the TG analysis, the RGO content in the tin oxide/RGO composite powders was estimated to be 4 wt %. The weight decrease in the tin oxide/RGO/carbon composite powders caused by the decomposition of carbon and RGO was compensated somewhat by the weight increase caused by the oxidation of SnO to SnO2. Therefore, the minimum carbon and RGO content in the tin oxide/RGO/carbon composite powders was estimated to be 17 wt %. Figure 3 d shows
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Full Paper RGO, tin oxide/carbon, and bare tin oxide powders were 1543, 1558, 1279, and 1484 mA h g 1, respectively. Figure 4 c shows the cycling performances of the four samples at a constant current density of 1000 mA g 1. The discharge capacities of the tin oxide/RGO/carbon, tin oxide/ RGO, tin oxide/carbon, and bare tin oxide powders after 175 cycles were 844, 331, 175, and 91 mA h g 1, respectively, and their capacity retentions measured from the second cycles were 80, 28, 19, and 9 %. All impedance measurements were performed at room temperature after 50 cycles at a current density of 1000 mA g 1. The resulting Nyquist plots (Figure 4 d) show a semicircle in the medium frequency region, assigned to the charge-transfer resistance of the Figure 3. a) XRD patterns, b) pore-size distributions, c) TG curves, and d) Raman spectra of the powders prepared electrodes, and a line inclined to from the various types of spray solutions. the real axis at low frequencies, which corresponds to lithium diffusion within the electrodes.[39–41] The tin oxide/RGO/carbon Raman spectra of the tin oxide/RGO/carbon composite and bare SnO2 powders. Two characteristic D and G graphene composite powders have the lowest charge-transfer resistance after cycling, as shown in Figure 4 d. The relationship between bands are observed at 1323 and 1588 cm 1, respectively, for the real part of the impedance spectra (Zre) and w 1/2 (w is the the composite powders, which are not observed in the spectrum of the bare SnO2 powders. In the preparation of tin angular frequency, w = 2pf) in the low-frequency region, as shown in Figure 4 e, reveals a higher lithium-ion diffusion rate oxide/RGO/carbon composite powders by one-pot spray pyrolfor the tin oxide/RGO/carbon composite powders than for the ysis, RGO is formed by the thermal reduction of graphene other samples.[40, 41] These results are attributable to the strucoxide sheets.[35, 36] The cyclic voltammograms (CVs) obtained for the tin oxide/ tural stability of the tin oxide/RGO/carbon composite powders RGO/carbon composite powders for the first ten cycles at during cycling, which lowers the charge-transfer resistance and a scan rate of 0.07 mV s 1 are shown in Figure 4 a. The CV thereby improves the lithium-ion diffusion rate. Tin oxide/RGO/ curve for the first discharging process shows two reduction carbon composite powders with high structural stability have peaks, at 0.16 and 0.87 V. The reduction peak at 0.87 V is assobetter rate performances than bare tin oxide powders, as ciated with the formation of metallic Sn nanograins and amorshown in Figure 4 f. TEM images of the tin oxide/RGO/carbon phous Li2O through the reduction of SnO2 and SnO.[18] The recomposite and bare tin oxide powders after 100 cycles are duction peak at 0.16 V is attributed to the formation of LixSn shown in Figure 5. After cycling, the spherical morphology of alloys.[37] The strong broad peak at approximately 0.52 V in the the particles is lost for the bare tin oxide powders. However, first charging process is attributed to Li dealloying from the tin oxide/RGO/carbon composites retained their spherical LixSn.[18] The reduction peak at 0.87 V disappeared after the morphology after cycling. Hollow nanovoids distributed uniformly within the composite powder are indicated by the first cycle, indicating that only the reversible conversion of Sn arrows in the TEM images, and are the result of the transforto LixSn was allowed in subsequent cycles.[37] The good overlap mation during cycling of SnO nanoflakes, uniformly dispersed with the CV curves from the second cycle reveals the reversibilin the tin oxide/RGO/carbon composite powder, into ultrafine ity of the process within the tin oxide/RGO/carbon composite nanocrystals. The high-resolution TEM images shown in Figpowders. The initial charge and discharge curves at a current ure 5 b and 5 d reveal the amorphous-like structure of the density of 1000 mA g 1, shown in Figure 4 b, have similar powder particles after cycling. The hollow nanovoids act as shapes for the four samples. The potential plateaus in the inibuffers for the large volume changes that occur during cycling, tial discharge curves at approximately 1.0 V are related to the such that the tin oxide/RGO/carbon composite powders have reduction of SnO and SnO2 to metallic Sn nanograins and superior cycling and rate performances. amorphous Li2O, as described for the CV results.[26] The initial discharge capacities of the tin oxide/RGO/carbon, tin oxide/ Chem. Eur. J. 2014, 20, 15203 – 15207
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Full Paper Experimental Section Synthesis of powders Graphene oxide (GO) was synthesized from graphite flakes following a modified Hummers The as-obtained method.[35, 38] graphite oxide was redispersed in distilled water and exfoliated ultrasonically to generate graphene oxide sheets. The graphene oxide sheets thus obtained were used to prepare the spray solution by dissolving tin oxalate (SnC2O4 ; 5.3 g) in exfoliated graphene oxide dispersion (400 mL; 1 mg mL 1). The tin oxide/RGO/carbon composite powders were prepared directly by spray pyrolysis from the colloidal spray solution containing GO, sucrose, and tin oxalate. Tin oxide/ graphene composite powders were prepared from the above colloidal spray solution without sucrose. Tin oxide/carbon and bare tin oxide powders were prepared from the spray solutions with and without sucrose, respectively. The ultrasonic spray pyrolysis system used in this study was described in our previous articles.[43, 44] In the experiment, a quartz reactor of 1200 mm in length and 50 mm in diameter was maintained at 800 8C. The nitrogen flow rate (carrier gas) during spray pyrolysis was maintained at 10 L min 1. Figure 4. Electrochemical properties of the four samples: a) CV curves of the tin oxide/RGO/carbon, b) initial charge and discharge curves, c) cycling performances, d) Nyquist plots after 50 cycles, e) relationships between the real part of the impedance (Zre) and w 1/2, and f) rate performances.
Conclusion The electrochemical properties of tin oxide/RGO/carbon composite powders as anode materials for LIBs were compared to those of tin oxide/RGO, tin oxide/carbon, and bare tin oxide powders prepared through the same process. Sucrose dissolved into the spray solution as the carbon source material enabled the formation of hierarchically structured tin oxide/ RGO/carbon composite powders. The reducing atmosphere around the powder particles caused by carbon material formed by the carbonization of sucrose produced SnO nanoflakes, which were dispersed uniformly throughout the tin oxide/RGO/carbon composite powder. The tin oxide/RGO/ carbon composite powders, with a mixed crystal structure of SnO and SnO2, had superior electrochemical properties to those of tin oxide/RGO, tin oxide/carbon, and bare tin oxide powders. Hollow nanovoids formed during cycling by the transformation of SnO nanoflakes into ultrafine nanocrystals improved the electrochemical properties of the tin oxide/RGO/ carbon composite powders. Chem. Eur. J. 2014, 20, 15203 – 15207
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Characterization
The crystal structures of the prepared powders were investigated by X-ray diffraction (XRD, X’pert PRO MPD) using CuKa radiation (l = 1.5418 ) at the Korea Basic Science Institute (Daegu). The morphologies of the prepared composites were observed by scanning electron microscopy (SEM, JEOL JSM-6060) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F) operating at a voltage of 200 kV. The specific surface areas of the powders were calculated from nitrogen adsorption measurements (TriStar 3000) using the Brunauer–Emmett–Teller (BET) method. Raman spectroscopic measurements were performed on a Nanofinder 30 spectrometer (Tokyo Instruments) using a He-Ne laser with a wavelength of 633 nm.
Electrochemical measurements The capacity and cycling properties of the prepared powders were determined by constructing a 2032-type coin cell. The electrode was prepared from a mixture containing 70 wt % active material, 20 wt % Super P, and 10 wt % sodium carboxymethyl cellulose (CMC) binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was composed of 1 m LiPF6 in a mixture of ethylene
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Figure 5. Morphologies of a,b) bare tin oxide and c,d) tin oxide/RGO/carbon composite after 100 cycles.
carbonate/dimethyl carbonate (EC/DMC; 1:1 by volume) containing 2 % vinylene carbonate. The charge/discharge characteristics of the samples were determined by cycling in the voltage range 0.001– 3 V. Cyclic voltammetry measurements were performed at a scan rate of 0.07 mV s 1.
Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A02046367). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (201320200000420). Keywords: energy conversion · energy-storage materials · graphene · nanostructures · synthesis design [1] L. Zhang, H. Wu, B. Liu, X. W. Lou, Energy Environ. Sci. 2014, 7, 1013 – 1017. [2] L. Zhang, G. Zhang, H. B. Wu, L. Yu, X. W. Lou, Adv. Mater. 2013, 25, 2589 – 2593. [3] J. S. Chen, X. W. Lou, Small 2013, 9, 1877 – 1893. [4] Z. H. Wen, Q. Wang, Q. Zhang, J. H. Li, Adv. Funct. Mater. 2007, 17, 2772 – 2778. [5] X. W. Lou, C. M. Li, L. A. Archer, Adv. Mater. 2009, 21, 2536 – 2539. [6] C. Wang, Y. Zhou, M. Y. Ge, X. B. Xu, Z. L. Zhang, J. Z. Jiang, J. Am. Chem. Soc. 2010, 132, 46 – 47. [7] Y. Wang, H. C. Zeng, J. Y. Lee, Adv. Mater. 2006, 18, 645 – 649.
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Received: June 23, 2014 Published online on September 29, 2014
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