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Electrochemical properties of ultrafine Sb nanocrystals embedded in carbon microspheres for use as Na-ion battery anode materials†
Received 9th July 2014, Accepted 28th August 2014
You Na Ko and Yun Chan Kang*
DOI: 10.1039/c4cc05275g www.rsc.org/chemcomm
Ultrafine Sb nanocrystals, uniformly distributed in a carbon matrix with a microspherical morphology, were synthesized by one-pot spray pyrolysis. The Sb–carbon composite microspheres exhibited good Na-storage properties with stable cyclability, a capacity retention of 90% over 100 cycles, and good rate performance.
Since batteries are used increasingly for energy storage in largescale devices, such as electrical vehicles (EVs) and hybrid electrical vehicles (HEVs), much effort has been focused on developing batteries with higher electrochemical performances. Recently, Na-ion batteries (NIBs) have gained much attention as possible alternatives to Li-ion batteries (LIBs) because of their low cost, small environmental impact, and the natural abundance of Na.1–7 Because Na has a larger ionic radius than Li, finding Na-ion electrode materials with excellent kinetics and high capacities is a major obstacle for NIB development.6–9 Of the many prospective anode materials, metals alloying with Na such as Sn and Ge have attracted considerable attention.10–14 However, these metals suffer from large volume changes during alloying and dealloying processes.15,16 These volume changes lead to electrode pulverization and result in decreased electrical contact between active materials and reduced cycle life. These volume change obstacles, which prevent the practical application of such metals alloying with Na as anode materials, can be overcome by designing alloys with carbon nanocomposite architectures. Recently, Sb has been considered to be a promising candidate for NIBs because it has a high capacity, 660 mA h g 1, and experiences less volume expansion than other metals that can be alloyed with Na.17,18 Some approaches to the synthesis of Sb–carbon composite materials and their Na-storage properties have been reported.19–22 However, to the best of our knowledge, Sb–carbon composite materials with spherical morphologies have not yet been synthesized
Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea. E-mail: [email protected]; Fax: +82-2-928-3584 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc05275g
12322 | Chem. Commun., 2014, 50, 12322--12324
for NIB applications. The morphology of the electrode material is considered an important factor in achieving high electrochemical performance and tap density. Using electrode materials with spherical morphologies is one possible strategy being used to improve packing densities and simplify handling during the electrode manufacturing process.23,24 Thus, there has been growing interest in synthesizing electrode materials with spherical morphologies. The spray pyrolysis process is a facile and cost-effective method that has been widely applied to prepare microspherical carbon composite powders with various compositions.25–29 In this study, Sb–carbon composite microspheres were synthesized using a spray pyrolysis process and their electrochemical performances as anode material for NIBs were investigated. Ultrafine Sb nanocrystals were embedded in carbon matrices, which allowed for good electrochemical stability because carbon matrices can accommodate volume changes, prevent particle aggregation during cycling, and improve electrical conductivities. The crystal structure of the powders directly prepared by spray pyrolysis is shown in Fig. S1 (see ESI†). All of the diffraction peaks of the powders are in good agreement with the hexagonal Sb structure (JCPDS No. 35-0732). No impurity diffraction peaks, such as antimony oxides, were observed, suggesting complete decomposition of antimony chloride to Sb metal even at a short reaction time of 2.2 s for the powders inside the reactor maintained at 700 1C. The broad peak seen near 281, shown in the inset image of Fig. S1 (see ESI†), indicates the presence of amorphous carbon, which was formed by the carbonization of sucrose. From XRD patterns, it was confirmed that Sb–carbon composite powders were directly prepared by spray pyrolysis. The morphologies of the Sb–carbon composite microspheres directly prepared by spray pyrolysis are shown in Fig. 1. The SEM image of the Sb–carbon composite powder shown in Fig. 1a demonstrates a spherical particle morphology and an average particle size of 0.5 mm. From the TEM images (Fig. 1b and c), it can be clearly seen that the Sb–carbon composite powders had spherical shape with non-aggregation characteristics. Fig. 1d shows a magnified TEM image of the edge of a single particle
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Fig. 2 Electrochemical properties of the Sb–carbon composite microspheres and coarse Sb powders: (a) CVs of the Sb–carbon composite microspheres, (b) initial charge and discharge curves, (c) cycling performances, and (d) rate performances.
Fig. 1 Morphologies and elemental mapping images of the Sb–carbon composite microspheres: (a) SEM image, (b) and (c) TEM images, (d) high resolution TEM image, and (e) elemental mapping images.
shown in Fig. 1c. The high-resolution TEM image reveals that the microsphere was composed of ultrafine Sb nanocrystals embedded in an amorphous carbon matrix. The elemental mapping images presented in Fig. 1e show that the Sb and carbon components were uniformly distributed throughout the microspheres without element segregation. Energy dispersive X-ray spectroscopy (EDS) analysis (Fig. S2, see ESI†) indicated that the carbon content of the Sb–carbon composite microsphere powder was approximately 54 wt%. The electrochemical properties of the Sb–carbon composite microspheres were compared to those of the coarse Sb powders in a voltage range of 0.001–2 V vs. Na/Na+. Fig. 2a shows the first five cycles of the cyclic voltammograms (CVs) obtained for Sb– carbon composite microspheres. As shown in Fig. 2a, there was a strong cathodic peak at 0.31 V for the initial cathodic scan, which could be attributed to the formation of the Na–Sb alloy, Na3Sb.17,18 During subsequent cathodic scans, two reduction peaks were observed, at 0.67 and 0.49 V, corresponding to the multi-step transformation of amorphous Sb into hexagonal Na3Sb by Na-ion insertion.17,18 Darwiche et al.17 reported that crystalline Sb changed into amorphous Sb during the first cycle and that the amorphous Sb was subsequently transformed into hexagonal Na3Sb through a cubic Na3Sb intermediate during subsequent discharge cycles. The reduction peak at 0.33 V, observed in the second cathodic scan and absent in subsequent cycles, was the result of residual Sb that did not completely electrochemically react during the first discharge process because of kinetic limitations.17 Anodic peaks, which correspond to the Na-ion deinsertion from Na3Sb that results in amorphous Sb, were observed at 0.87 V during
This journal is © The Royal Society of Chemistry 2014
all of the charge scans. In contrast to those of the coarse Sb powders (Fig. S3, see ESI†), from the third cycle onward, the peaks of the Sb– carbon composite microsphere CV scans overlap substantially, indicating the outstanding charge and discharge cyclability of the Sb–carbon composite microspheres. The initial charge and discharge curves of the Sb–carbon composite microspheres obtained at a current density of 0.3 A g 1 are shown in Fig. 2b. The plateaus seen in the charge and discharge curves are consistent with the peaks seen in the CV curves. The Sb–carbon composite microspheres exhibited initial discharge and charge capacities of 625 and 402 mA h g 1, respectively. However, the coarse Sb powders exhibited initial discharge and charge capacities of 690 and 510 mA h g 1, respectively. The high carbon contents of the Sb–carbon composite microspheres lead to a decrease in the masses of active Sb materials and result in low initial capacities. The cycling performances of the Sb–carbon composite microspheres and coarse Sb powders were investigated using a constant current density of 0.3 A g 1, as shown in Fig. 2c. The discharge capacities of the Sb–carbon composite microspheres and coarse Sb powders were 372 and 46 mA h g 1, respectively, after 100 cycles. The Sb–carbon composite microspheres exhibited good cycling performance with a capacity retention of 90% which was measured after the first cycle because the carbon matrix in the microspheres could improve their structural stabilities by acting as a buffer layer to accommodate a volume change during Na-ion insertion and deinsertion. The rate performances of the Sb–carbon composite microspheres and coarse Sb powders were investigated using various current densities, which we increased stepwise from 0.2 to 3 A g 1, as shown in Fig. 2d. For each step, 10 cycles were measured in order to evaluate rate performance. The Sb–carbon composite microspheres exhibited high 10th discharge capacities of 395, 365, 340, 305, and 267 mA h g 1 at current densities of 0.2, 0.5, 1, 2, and 3 A g 1, respectively. When the current density was returned to 0.2 A g 1, the discharge capacity recovered to above 390 mA h g 1, similar to those of the first few cycles.
Chem. Commun., 2014, 50, 12322--12324 | 12323
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electrical conductivity, improving the electrochemical properties of the Sb–carbon composite microspheres. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2012R1A2A2A02046367).
Notes and references Fig. 3 Nyquist plots of the electrochemical impedance spectra of the Sb–carbon composite microspheres and coarse Sb powders: (a) before and (b) after cycling.
Impedance spectra obtained both before and after 100 cycles from Sb–carbon composite microspheres and coarse Sb powders are shown in Fig. 3. The Nyquist plots indicate compressed semicircles in the medium frequency range of each spectrum, which describe the charge transfer resistances (Rct) for these electrodes, and straight lines in the low frequency range, which are associated with Na-ion diffusion in the bulk of the active materials.30–33 After cycling, the radii of the semicircles of both the Sb–carbon composite microspheres and the coarse Sb powders decreased because the crystalline Sb of the samples converted to amorphous Sb during Na-ion insertion and deinsertion. In contrast to the coarse Sb powders, the Sb–carbon composite microspheres demonstrated low charge transfer resistances both before and after cycling and maintained straight lines in the low frequency region after cycling. The Sb–carbon composite microspheres with nanostructures and high electrical conductivities had better cycling and rate performances than did the coarse Sb powders. The good overall electrochemical performance of the Sb–carbon composite microspheres is attributed to their fine Sb particles being uniformly dispersed throughout their carbon matrices. The morphologies of the Sb–carbon composite microsphere and coarse Sb powder after 100 cycles were investigated (Fig. S4, see ESI†). The Sb–carbon composite microsphere maintained the spherical morphology even after 100 cycles. On the other hand, the original shape of the coarse Sb powder was destroyed after cycling. The repeated alloying and dealloying processes destroyed the structure of the coarse Sb powders. However, the amorphous carbon matrix accommodated the volume change of Sb nanocrystals during the repeated alloying and dealloying processes. The structural stability of the Sb–carbon composite microsphere during cycling resulted in the superior electrochemical properties of anode materials for sodium ion storage. In summary, we have demonstrated the synthesis of Sb–carbon composite microspheres with superior electrochemical properties for use as anode materials in NIBs. The Sb–carbon composite microspheres had better cycling and rate performances than the coarse Sb powders. The amorphous carbon matrix acted as a volume expansion buffer layer during cycling and improved
12324 | Chem. Commun., 2014, 50, 12322--12324
1 M. D. Slater, D. Kim, E. Lee and C. S. Johnson, Adv. Funct. Mater., 2013, 23, 947–958. 2 H. Pan, Y. S. Hu and L. Chen, Energy Environ. Sci., 2013, 6, 2338–2360. 3 B. L. Ellis and L. F. Nazar, Curr. Opin. Solid State Mater. Sci., 2012, 16, 168–177. 4 V. Palomares, M. Casas-Cabanas, E. Castillo-Martı´nez, M. H. Han and T. Rojo, Energy Environ. Sci., 2013, 6, 2312–2337. 5 V. L. Chevrier and G. Ceder, J. Electrochem. Soc., 2011, 158, A1011–A1014. ´lez 6 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonza and T. Rojo, Energy Environ. Sci., 2012, 5, 5884–5901. 7 S. W. Kim, D. H. Seo, X. Ma, G. Ceder and K. Kang, Adv. Energy Mater., 2012, 2, 710–721. 8 S. Y. Hong, Y. Kim, Y. Park, A. Choi, N. S. Choi and K. T. Lee, Energy Environ. Sci., 2013, 6, 2067–2081. 9 L. Wang, K. Zhang, Z. Hu, W. Duan, F. Cheng and J. Chen, Nano Res., 2014, 7, 199–208. 10 Y. Xu, Y. Zhu, Y. Liu and C. Wang, Adv. Energy Mater., 2013, 3, 128–133. 11 H. Zhu, Z. Jia, Y. Chen, N. Weadock, J. Wan, O. Vaaland, X. Han, T. Li and L. Hu, Nano Lett., 2013, 13, 3093–3100. 12 M. K. Datta, R. Epur, P. Saha, K. Kadakia, S. K. Park and P. N. Kumta, J. Power Sources, 2013, 225, 316–322. 13 P. R. Abel, Y. M. Lin, T. de Souza, C. Y. Chou, A. Gupta, J. B. Goodenough, G. S. Hwang, A. Heller and C. B. Mullins, J. Phys. Chem. C, 2013, 117, 18885–18890. 14 L. Baggetto, J. K. Keum, J. F. Browning and G. M. Veith, Electrochem. Commun., 2013, 34, 41–44. 15 J. W. Wang, X. H. Liu, S. X. Mao and J. Y. Huang, Nano Lett., 2012, 12, 5897–5902. 16 J. S. Chen and X. W. Lou, Small, 2013, 9, 1877–1893. 17 A. Darwiche, C. Marino, M. T. Sougrati, B. Fraisse, L. Stievano and L. Monconduit, J. Am. Chem. Soc., 2012, 134, 20805–20811. 18 M. He, K. Kravchyk, M. Walter and M. V. Kovalenko, Nano Lett., 2014, 14, 1255–1262. 19 J. Qian, Y. Chen, L. Wu, Y. Cao, X. Ai and H. Yang, Chem. Commun., 2012, 48, 7070–7072. 20 L. Wu, X. Hu, J. Qian, F. Pei, F. Wu, R. Mao, X. Ai, H. Yang and Y. Cao, Energy Environ. Sci., 2014, 7, 323–328. 21 Y. Zhu, X. Han, Y. Xu, Y. Liu, S. Zheng, K. Xu, L. Hu and C. Wang, ACS Nano, 2013, 7, 6378–6386. 22 W. Luo, S. Lorger, B. Wang, C. Bommier and X. Ji, Chem. Commun., 2014, 50, 5435–5437. 23 W. Liu, P. Gao, Y. Mi, J. Chen, H. Zhou and X. Zhang, J. Mater. Chem. A, 2013, 1, 2411–2417. 24 X. Xiao, J. Lu and Y. Li, Nano Res., 2010, 3, 733–737. 25 Y. N. Ko, S. B. Park and Y. C. Kang, Small, 2014, 10, 3240–3245. 26 S. H. Choi and Y. C. Kang, ChemSusChem, 2014, 7, 523–528. 27 Y. N. Ko, H. Y. Koo, J. H. Kim, J. H. Yi, Y. C. Kang and J. H. Lee, J. Power Sources, 2011, 196, 6682–6687. 28 J. Guo, Q. Liu, C. Wang and M. R. Zachariah, Adv. Funct. Mater., 2012, 22, 803–811. 29 Y. Xu, Q. Liu, Y. Zhu, Y. Liu, A. Langrock, M. R. Zachariah and C. Wang, Nano Lett., 2013, 13, 470–474. 30 S. Yang, H. Song and X. Chen, Electrochem. Commun., 2006, 8, 137–142. 31 Y. N. Ko, S. B. Park, K. Y. Jung and Y. C. Kang, Nano Lett., 2013, 13, 5462–5466. 32 Y. Shi, J. Z. Wang, S. L. Chou, D. Wexler, H. J. Li, K. Ozawa, H. K. Liu and Y. P. Wu, Nano Lett., 2013, 13, 4715–4720. 33 S. H. Choi and Y. C. Kang, ChemSusChem, 2013, 6, 2111–2116.
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Cite this: Chem. Commun., 2014, 50, 12322
Electrochemical properties of ultrafine Sb nanocrystals embedded in carbon microspheres for use as Na-ion battery anode materials†
Received 9th July 2014, Accepted 28th August 2014
You Na Ko and Yun Chan Kang*
DOI: 10.1039/c4cc05275g www.rsc.org/chemcomm
Ultrafine Sb nanocrystals, uniformly distributed in a carbon matrix with a microspherical morphology, were synthesized by one-pot spray pyrolysis. The Sb–carbon composite microspheres exhibited good Na-storage properties with stable cyclability, a capacity retention of 90% over 100 cycles, and good rate performance.
Since batteries are used increasingly for energy storage in largescale devices, such as electrical vehicles (EVs) and hybrid electrical vehicles (HEVs), much effort has been focused on developing batteries with higher electrochemical performances. Recently, Na-ion batteries (NIBs) have gained much attention as possible alternatives to Li-ion batteries (LIBs) because of their low cost, small environmental impact, and the natural abundance of Na.1–7 Because Na has a larger ionic radius than Li, finding Na-ion electrode materials with excellent kinetics and high capacities is a major obstacle for NIB development.6–9 Of the many prospective anode materials, metals alloying with Na such as Sn and Ge have attracted considerable attention.10–14 However, these metals suffer from large volume changes during alloying and dealloying processes.15,16 These volume changes lead to electrode pulverization and result in decreased electrical contact between active materials and reduced cycle life. These volume change obstacles, which prevent the practical application of such metals alloying with Na as anode materials, can be overcome by designing alloys with carbon nanocomposite architectures. Recently, Sb has been considered to be a promising candidate for NIBs because it has a high capacity, 660 mA h g 1, and experiences less volume expansion than other metals that can be alloyed with Na.17,18 Some approaches to the synthesis of Sb–carbon composite materials and their Na-storage properties have been reported.19–22 However, to the best of our knowledge, Sb–carbon composite materials with spherical morphologies have not yet been synthesized
Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea. E-mail: [email protected]; Fax: +82-2-928-3584 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc05275g
12322 | Chem. Commun., 2014, 50, 12322--12324
for NIB applications. The morphology of the electrode material is considered an important factor in achieving high electrochemical performance and tap density. Using electrode materials with spherical morphologies is one possible strategy being used to improve packing densities and simplify handling during the electrode manufacturing process.23,24 Thus, there has been growing interest in synthesizing electrode materials with spherical morphologies. The spray pyrolysis process is a facile and cost-effective method that has been widely applied to prepare microspherical carbon composite powders with various compositions.25–29 In this study, Sb–carbon composite microspheres were synthesized using a spray pyrolysis process and their electrochemical performances as anode material for NIBs were investigated. Ultrafine Sb nanocrystals were embedded in carbon matrices, which allowed for good electrochemical stability because carbon matrices can accommodate volume changes, prevent particle aggregation during cycling, and improve electrical conductivities. The crystal structure of the powders directly prepared by spray pyrolysis is shown in Fig. S1 (see ESI†). All of the diffraction peaks of the powders are in good agreement with the hexagonal Sb structure (JCPDS No. 35-0732). No impurity diffraction peaks, such as antimony oxides, were observed, suggesting complete decomposition of antimony chloride to Sb metal even at a short reaction time of 2.2 s for the powders inside the reactor maintained at 700 1C. The broad peak seen near 281, shown in the inset image of Fig. S1 (see ESI†), indicates the presence of amorphous carbon, which was formed by the carbonization of sucrose. From XRD patterns, it was confirmed that Sb–carbon composite powders were directly prepared by spray pyrolysis. The morphologies of the Sb–carbon composite microspheres directly prepared by spray pyrolysis are shown in Fig. 1. The SEM image of the Sb–carbon composite powder shown in Fig. 1a demonstrates a spherical particle morphology and an average particle size of 0.5 mm. From the TEM images (Fig. 1b and c), it can be clearly seen that the Sb–carbon composite powders had spherical shape with non-aggregation characteristics. Fig. 1d shows a magnified TEM image of the edge of a single particle
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Fig. 2 Electrochemical properties of the Sb–carbon composite microspheres and coarse Sb powders: (a) CVs of the Sb–carbon composite microspheres, (b) initial charge and discharge curves, (c) cycling performances, and (d) rate performances.
Fig. 1 Morphologies and elemental mapping images of the Sb–carbon composite microspheres: (a) SEM image, (b) and (c) TEM images, (d) high resolution TEM image, and (e) elemental mapping images.
shown in Fig. 1c. The high-resolution TEM image reveals that the microsphere was composed of ultrafine Sb nanocrystals embedded in an amorphous carbon matrix. The elemental mapping images presented in Fig. 1e show that the Sb and carbon components were uniformly distributed throughout the microspheres without element segregation. Energy dispersive X-ray spectroscopy (EDS) analysis (Fig. S2, see ESI†) indicated that the carbon content of the Sb–carbon composite microsphere powder was approximately 54 wt%. The electrochemical properties of the Sb–carbon composite microspheres were compared to those of the coarse Sb powders in a voltage range of 0.001–2 V vs. Na/Na+. Fig. 2a shows the first five cycles of the cyclic voltammograms (CVs) obtained for Sb– carbon composite microspheres. As shown in Fig. 2a, there was a strong cathodic peak at 0.31 V for the initial cathodic scan, which could be attributed to the formation of the Na–Sb alloy, Na3Sb.17,18 During subsequent cathodic scans, two reduction peaks were observed, at 0.67 and 0.49 V, corresponding to the multi-step transformation of amorphous Sb into hexagonal Na3Sb by Na-ion insertion.17,18 Darwiche et al.17 reported that crystalline Sb changed into amorphous Sb during the first cycle and that the amorphous Sb was subsequently transformed into hexagonal Na3Sb through a cubic Na3Sb intermediate during subsequent discharge cycles. The reduction peak at 0.33 V, observed in the second cathodic scan and absent in subsequent cycles, was the result of residual Sb that did not completely electrochemically react during the first discharge process because of kinetic limitations.17 Anodic peaks, which correspond to the Na-ion deinsertion from Na3Sb that results in amorphous Sb, were observed at 0.87 V during
This journal is © The Royal Society of Chemistry 2014
all of the charge scans. In contrast to those of the coarse Sb powders (Fig. S3, see ESI†), from the third cycle onward, the peaks of the Sb– carbon composite microsphere CV scans overlap substantially, indicating the outstanding charge and discharge cyclability of the Sb–carbon composite microspheres. The initial charge and discharge curves of the Sb–carbon composite microspheres obtained at a current density of 0.3 A g 1 are shown in Fig. 2b. The plateaus seen in the charge and discharge curves are consistent with the peaks seen in the CV curves. The Sb–carbon composite microspheres exhibited initial discharge and charge capacities of 625 and 402 mA h g 1, respectively. However, the coarse Sb powders exhibited initial discharge and charge capacities of 690 and 510 mA h g 1, respectively. The high carbon contents of the Sb–carbon composite microspheres lead to a decrease in the masses of active Sb materials and result in low initial capacities. The cycling performances of the Sb–carbon composite microspheres and coarse Sb powders were investigated using a constant current density of 0.3 A g 1, as shown in Fig. 2c. The discharge capacities of the Sb–carbon composite microspheres and coarse Sb powders were 372 and 46 mA h g 1, respectively, after 100 cycles. The Sb–carbon composite microspheres exhibited good cycling performance with a capacity retention of 90% which was measured after the first cycle because the carbon matrix in the microspheres could improve their structural stabilities by acting as a buffer layer to accommodate a volume change during Na-ion insertion and deinsertion. The rate performances of the Sb–carbon composite microspheres and coarse Sb powders were investigated using various current densities, which we increased stepwise from 0.2 to 3 A g 1, as shown in Fig. 2d. For each step, 10 cycles were measured in order to evaluate rate performance. The Sb–carbon composite microspheres exhibited high 10th discharge capacities of 395, 365, 340, 305, and 267 mA h g 1 at current densities of 0.2, 0.5, 1, 2, and 3 A g 1, respectively. When the current density was returned to 0.2 A g 1, the discharge capacity recovered to above 390 mA h g 1, similar to those of the first few cycles.
Chem. Commun., 2014, 50, 12322--12324 | 12323
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electrical conductivity, improving the electrochemical properties of the Sb–carbon composite microspheres. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2012R1A2A2A02046367).
Notes and references Fig. 3 Nyquist plots of the electrochemical impedance spectra of the Sb–carbon composite microspheres and coarse Sb powders: (a) before and (b) after cycling.
Impedance spectra obtained both before and after 100 cycles from Sb–carbon composite microspheres and coarse Sb powders are shown in Fig. 3. The Nyquist plots indicate compressed semicircles in the medium frequency range of each spectrum, which describe the charge transfer resistances (Rct) for these electrodes, and straight lines in the low frequency range, which are associated with Na-ion diffusion in the bulk of the active materials.30–33 After cycling, the radii of the semicircles of both the Sb–carbon composite microspheres and the coarse Sb powders decreased because the crystalline Sb of the samples converted to amorphous Sb during Na-ion insertion and deinsertion. In contrast to the coarse Sb powders, the Sb–carbon composite microspheres demonstrated low charge transfer resistances both before and after cycling and maintained straight lines in the low frequency region after cycling. The Sb–carbon composite microspheres with nanostructures and high electrical conductivities had better cycling and rate performances than did the coarse Sb powders. The good overall electrochemical performance of the Sb–carbon composite microspheres is attributed to their fine Sb particles being uniformly dispersed throughout their carbon matrices. The morphologies of the Sb–carbon composite microsphere and coarse Sb powder after 100 cycles were investigated (Fig. S4, see ESI†). The Sb–carbon composite microsphere maintained the spherical morphology even after 100 cycles. On the other hand, the original shape of the coarse Sb powder was destroyed after cycling. The repeated alloying and dealloying processes destroyed the structure of the coarse Sb powders. However, the amorphous carbon matrix accommodated the volume change of Sb nanocrystals during the repeated alloying and dealloying processes. The structural stability of the Sb–carbon composite microsphere during cycling resulted in the superior electrochemical properties of anode materials for sodium ion storage. In summary, we have demonstrated the synthesis of Sb–carbon composite microspheres with superior electrochemical properties for use as anode materials in NIBs. The Sb–carbon composite microspheres had better cycling and rate performances than the coarse Sb powders. The amorphous carbon matrix acted as a volume expansion buffer layer during cycling and improved
12324 | Chem. Commun., 2014, 50, 12322--12324
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