Sn/In2O3/C Nanocomposite as an Anode for Li Ion Batteries and Its ...

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Journal of The Electrochemical Society, 159 (11) A1912-A1915 (2012) 0013-4651/2012/159(11)/A1912/4/$28.00 © The Electrochemical Society

Sn/In2 O3 /C Nanocomposite as an Anode for Li Ion Batteries and Its Reaction Mechanism Won-Seok Chang,a Byeong-Chul Yu,a Cheol-Min Park,b and Hun-Joon Sohna,∗,z a Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea b School of Advanced Materials & System Engineering, Kumoh National Institute of Technology, Gumi

730-701, Korea

Sn/In2 O3 /C nano composite was prepared from SnO, In and amorphous carbon (Super P) using thermal reduction followed by highenergy mechanical milling. The nanocomposite consisted of nanosize Sn and In2 O3 crystallites, which were randomly distributed in the amorphous carbon matrix. The electrochemical performances of the nanocomposite were evaluated and the reaction mechanism of the composite during cycling was investigated using an X-ray diffractometer (XRD) and high-resolution transmission electron microscopy (HRTEM). The cycle performance of the nanocomposite was greatly enhanced when compared to each component of the composite. The nanocomposite exhibited charge capacity of ca. 600 mAh g−1 over 100 cycles, with a first-cycle Coulombic efficiency of 71%. © 2012 The Electrochemical Society. [DOI: 10.1149/2.033212jes] All rights reserved. Manuscript submitted August 8, 2012; revised manuscript received August 31, 2012. Published September 24, 2012. This was Paper 1363 presented at the Boston, Massachusetts, Meeting of the Society, October 9–14, 2011.

Recently, Sn-based composites have drawn much interest along with Si-based composites for the anode materials of Li-ion batteries. Metallic Sn has a theoretical capacity of 993 mAh g−1 (Li22 Sn5 ).1–5 Even though, this capacity is larger compared to graphite (372 mAh g−1 , LiC6 ), the capacity sustainability of Sn alone during cycling was very poor because of a large volume variation when Li was inserted/extracted.6–9 To take advantage of the high capacity of Sn, many studies have been dedicated to the improvement of cycle performance.10–14 Among them, Sn-based composite using carbon or metal oxides has provided stability in electrochemical performance. Carbon surrounds Sn particles and efficiently acts as a buffering matrix against volume variation.15 Metal oxides have also been utilized as an active ceramic in the composite, which stabilized the cycle performance and reacted with Li simultaneously. For instance,16 although some TiO2 forms an irreversible LiTiO2 phase during initial lithiation, the remaining TiO2 reacts with Li reversibly with the cycle. These two phases contribute to improve the electrochemical performance by enhancing the mechanical properties of the composite. In2 O3 has also been investigated as an anode material for Li ion batteries, and has exhibited reversible behavior during electrochemical cycling.17–19 Approaches using favorable nanostructures such as nano-blade17 and nano-porous thin film18,19 have been performed to enhance the poor performance of the material originating from volume expansion and Li2 O formation. The electrochemical performances were improved, but the fabrication process to obtain those structures is complicated. Along with the buffering matrix, the size of the active material is also critical for the electrochemical performances as stated above. Thus, there have been many approaches to obtain nanosized anode materials,20–22 owing to benefits such as large interfacial area and shorter diffusion length for the Li ions. High-energy mechanical milling (HEMM) is a simple and quite attractive tool to produce nanosize powders among various synthesis procedures since the temperature and pressure can reach up to 200 ◦ C and 6 GPa,23 respectively. In this study, Sn/In2 O3 /C nanocomposite was synthesized by simple heat-treatment followed by HEMM, and its electrochemical performance and reaction mechanism were investigated. Experimental Sn/In2 O3 /C nanocomposite was prepared by the following procedures. SnO (Aldrich, >99%, 10 μm) and In (Aldrich, 99.99%) were mixed well, placed in a quartz tube, and heat-treated at 900◦ C for 6 h under Ar atmosphere. After thermal reduction process, the heattreated product and carbon black (Timcal, Super P) with a weight ∗ z

Electrochemical Society Active Member. E-mail: [email protected]

ratio of 60:40 was put into a steel vial (80 cm3 ) containing two different sizes of steel balls (3/8 in. and 16/3 in.). Preliminary experiments showed that the optimum weight ratio of heat-treated product to amorphous carbon was 60 to 40 for the best electrochemical performances. Assembly was carried out in an Ar-filled glove box, and high-energy mechanical milling (HEMM) was performed at 800 rpm for 6 h. To identify the product and investigate the reaction chemistry of Sn-In2 O3 -C composite during the Li insertion/extraction, X-ray diffraction (XRD, Rigaku D-MAX 2500) and high-resolution transmission electron microscopy (HRTEM, JEOL 3000F operating at 300 kV) were employed. For the preparation of ex-situ samples, the electrodes were taken off the cell and washed with diethyl carbonate (DEC), and dried in a vacuum chamber. The dried electrodes were sealed with polyimide tape to prevent contact with air. For the preparation of the HRTEM sample, the dried electrodes were put into anhydrous ethyl alcohol and sonicated. This dilute suspension of the sample was dropped onto a carbon-coated TEM grid and dried in a vacuum chamber for 3 h. To evaluate the electrochemical performance of the composite, electrodes were prepared from a slurry composed of active material (70 wt%), a binder (Polyvinylidene fluoride (PVDF)) (15 wt%) and a conducting agent (super P, 15 wt%). Slurry was made by mixing these 3 components in a solvent (N-methyl pyrrolidinone (NMP)), and the slurry was coated onto copper foil and dried in a vacuum oven at 120◦ C for 4 h. Coin-type electrochemical cells were assembled in an Arfilled glove box. Li foil was used as counter and reference electrodes, and Celgard 2400 as a separator. 1 M LiPF6 in ethylene carbonate (EC)/diethylene carbonate (DEC) (1:1 vol%, Panax Starlyte) was used as an electrolyte. All tests were performed galvanostatically within a voltage window of 0.0–2.0 V with a current of 100 mA g−1 . Li was inserted into the electrode during the discharge, and Li was extracted from the electrode during the charging step. Results and Discussion Sn-In2 O3 -C nanocomposite was prepared by facile reduction methods via the following reaction: (G◦ = −72 kJ mol−1 , 298 K) [1] Figure 1a shows the XRD pattern of Sn/In2 O3 /C nanocomposite compared with the precursors (SnO and In). All peaks corresponded to metallic Sn (PDF # 03-065-7657) and In2 O3 (PDF # 03-0653170), indicating that SnO was completely reduced to form metallic Sn and In2 O3 during heat-treatment. After the HEMM process, the Sn/In2 O3 /C nanocomposite composed of nanosized Sn and In2 O3 particles (ca. 20 nm) embedded randomly in the carbon matrix is shown in Figure 1b, as observed from high-resolution TEM with the fast Fourier transformed (FFT) images. 3 SnO + 2 In → 3 Sn + In2 O3

Journal of The Electrochemical Society, 159 (11) A1912-A1915 (2012)

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Figure 1. (a) XRD patterns of Sn-In2 O3 -C composite and powder diffraction patterns of references and (b) TEM image of Sn-In2 O3 -C nanocomposite.

The voltage profiles of metallic Sn, In, In2 O3 and Sn/In2 O3 /C nanocomposite are presented in Figure 2. The discharge and charge capacities of Sn during the first cycle were 675 and 504 mAh g−1 , respectively, but decreased rapidly due to the severe volume variation when cycled24,25 as shown in Figure 2a. Meanwhile, metallic In showed no reactivity with Li during cycling (Fig. 2b). On the other hand, In2 O3 exhibited a high discharge capacity of 1307 mAh g−1 at the first cycle, which also degraded drastically within a few cycles (Fig. 2c). The initial Coulombic efficiencies were 75% for the Sn and 68% for the In2 O3 . When these components were fabricated as a nanocomposite, the electrochemical performance was greatly enhanced as shown in Figure 2d. The Sn/In2 O3 /C nanocomposite exhibited discharge capacity of 1197 mAh g−1 and charge capacity of 852 mAh g−1 with an initial Coulombic efficiency of 71%. Assuming that the reaction 1 was completed (Sn : 34 wt%, In2 O3 : 26 wt%, C : 40 wt%), the calculated discharge capacity would be 1232 mAh g−1 which is close to the experimental value. The Coulombic efficiency was increased to 94% at the 2nd cycle and over 97% from the 10th cycle. The reaction mechanism of the Sn/In2 O3 /C composite electrode during the first cycle was investigated by ex-situ XRD and HRTEM methods. Figure 3a compares the differential capacity plots (DCPs) of the Sn/In2 O3 /C nanocomposite (black solid line) with Sn (red dotted line). The In2 O3 (blue dotted line) XRD patterns and HRTEM images of Sn/In2 O3 /C at the potentials indicated in the DCPs are presented in Figure 3b and Figure 4, respectively. The DCP of the Sn/In2 O3 /C nanocomposite showed a hump near 1.1 V, and a similar phenomenon was also exhibited from Sn-C composite due to catalytic electrolyte decomposition26,27 or SEI formation.28 The HRTEM image with diffraction patterns at 0.65 V during the discharge (Fig. 4a)

Figure 2. Voltage profiles of (a) metallic Sn (b) In (c) In2 O3 and (d) SnIn2 O3 -C nanocomposite.

revealed InLiO2 with In and Sn phases, although InLiO2 phase was not detected from the XRD pattern (Fig. 3b). The presence of InLiO2 phase at this potential indicated that the reaction between In2 O3 with Li occurred as: 2 In2 O3 + 3 Li+ + 3 e− ↔ 3 InLiO2 + In

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Figure 3. (a) DCP of, Sn, In2 O3 , Sn-C composite and Sn-In2 O3 -C nanocomposite at the first cycle. (b) Corresponding ex-situ XRD patterns at the selected potentials in DCP. (Red : Sn, Purple : In, Blue : In2 O3 , Black : Li4.4 Sn, Green: LiInO2 ).

When the potential was lowered to 0.0 V, only In and Li22 Sn5 phases were observed as shown in Figure 4b, and LiInO2 decomposed into Li2 O and In as follows: 3 LiInO2 + 9 Li+ + 9 e− ↔ 6 Li2 O + 3 In

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5 Sn + 22 Li+ + 22 e− ↔ Li22 Sn5

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The Li2 O phase formed in the above reaction would be amorphous as in most other cases,29 and this agreed with the result given in the previous report on amorphous thin film In2 O3 .18,19 Although Li-In binary alloy phase such as InLi2 has been reported for In2 O3 nanoblade17 or amorphous thin film,18,19 Li-In alloy phase was not observed in this case, which agreed with the voltage profile (Fig. 2b) and the result by Kim et al.30 When the potential was increased to 0.85 V during the charge, the formation of InLiO2 and decomposition of Li22 Sn5 phase into metallic Sn could be observed as shown in Figure 4c. The peaks indicating Sn and In could hardly be resolved from XRD patterns when charged these phases were confirmed with HRTEM data. This indicated that the reverse reactions of reactions (3) and (4) occurred during the discharging step. The amorphous Li2 O reacted with nanosize In to form LiInO2 phase, similar to the transition metal oxides case reported by Poizot et al.22 When Li was fully extracted to 2.0 V, In2 O3 reappeared by the reverse reaction of 2 as shown in Figure 4d. The broad oxidation peak around 1.7 V in DCP would be related to the recovery of In2 O3 , as can be seen in Fig. 3a, which is in agreement with a previous report.18 However, some of the In phase still remained even after the

Figure 4. Ex-situ HRTEM images at the selected potentials (a) discharging step at 0.65 V, (b) discharging step at 0.0 V, (c) charging step at 0.85 V (d) charging step at 2.0 V.

full extraction of Li (Fig. 4d), indicating the incomplete recovery of In2 O3. This contributed to the irreversible capacity of the first cycle. Also, from the DCP of In2 O3 electrode (Fig. 2c), the plateau for reaction (2) near 0.75 V at the first discharge and corresponding plateau for the reverse of reaction (2) near 1.64 V during the first charge were shortened with the cycle, which was consistent with the above observation The cycle performances of various electrodes are compared in Figure 5. Metallic In electrode did not react with Li as shown in the figure, and the reversible charge capacities of pure Sn and In2 O3 electrodes faded rapidly. But the reversible capacity of the Sn/In2 O3 /C composite was maintained over 600 mAh g−1 even at


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Acknowledgments This work was supported by the research fund of the POSCO Chair Professor (H.-J. Sohn) at Seoul National University. References

Figure 5. Cycle performances of the metallic Sn and In, In2 O3 , Sn-In2 O3 -C nanocomposite.

the 100th cycle. The improvement in the electrochemical performance would be attributed to the randomly dispersed nanosize Sn and In2 O3 particles in the carbon matrix. In addition, the buffering effect of the amorphous carbon matrix was another factor contributing to the improvement; it enabled the active materials to react with Li by mitigating the volume expansion and contraction during cycling.7,16

Conclusions Sn/In2 O3 /C nanocomposite was prepared by simple heat-treatment and HEMM. When tested as an anode for Li rechargeable batteries, it exhibited not only better electrochemical performance compared to the graphite anode but also stabilized cycle performance. The reaction mechanism was suggested using XRD and HRTEM techniques. The enhancement in the electrochemical performance of the Sn/In2 O3 /C nanocomposite was mainly due that nanosized Sn and In2 O3 active materials dispersed randomly in the amorphous carbon matrix, as well as amorphous carbon matrix absorbing the stress induced by volume change.

1. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, and T. Miyasaka, Science, 276, 1395 (1997). 2. C. J. Wen and R. A. Huggins, J. Electrochem. Soc., 128, 1181 (1981). 3. R. A. Huggins, J. Power Sources, 81, 13 (1999). 4. X. W. Lou, Y. Wang, C. Yuan, J. Y. Lee, and L. A. Archer, Adv. Mater., 18, 2325 (2006). 5. G. Derrien, J. Hassoun, S. Panero, and B. Scrosati, Adv. Mater., 19, 2336 (2007). 6. G.-A. Nazri and G. Pistoia, Lithium Batteries; Science and Technology, Kluwer Academic/Plenum, Boston (2004). 7. C.-M. Park, J.-H. Kim, H. Kim, and H.-J. Sohn, Chem. Soc. Rev., 39, 3115 (2010). 8. M. Winter and J. O. Besenhard, Electrochim. Acta, 45, 31 (1999). 9. J.-H. Kim, G.-J. Jeong, Y.-W. Kim, H.-J. Sohn, C. W. Park, and C. K. Lee, J. Electrochem. Soc., 150, A1544 (2003). 10. J.-M. Lee, W.-S. Chang, B.-C. Yu, H. Kim, D. Im, S.-G. Doo, and H.-J. Sohn, Electrochem. Commun., 12, 928 (2010). 11. C.-M. Park and K.-J. Jeon, Chem. Commun., 47, 2122 (2011). 12. W.-R. Liu, Z.-Z. Guo, W.-S. Young, D.-T. Shieh, H.-C. Wu, M.-H. Yang, and N.-L. Wu, J. Power Sources, 140, 139 (2005). 13. J. Yao, X. Shen, B. Wang, H. Liu, and G. Wang, Electrochem. Commun., 11, 1849 (2009). 14. S. Ding, D. Luan, F. Y. C. Boey, J. S. Chen, and X. W. Lou, Chem. Commun., 47, 7155 (2011). 15. G. Derrien, J. Hassoun, S. Panero, and B. Scrosati, Adv. Mater., 19, 2336 (2007). 16. C.-M. Park, W.-S. Chang, H. C. Jung, J.-H. Kim, and H.-J. Sohn, Electrochem. Commun., 11, 2165 (2009). 17. R. Yang, J. Zheng, J. Huang, X. Z. Zhang, J. L. Qu, and X. G. Li, Electrochem. Commun., 12, 784 (2010). 18. Y. Zhou, H. Zuang, M. Xue, C. Wu, X. Wu, and Z. Fu, J. Power Sources, 162, 1373 (2006). 19. W.-H. Ho, C.-F. Li, H.-C. Liu, and S.-K. Yen, J. Power Sources, 175, 897 (2008). 20. A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, and W. V. Schalkwijk, Nat. Mater., 4, 366 (2005). 21. P. G. Bruce, B. Scrosati, and J.-M. Tarascon, Angew. Chem. Int. Ed., 47, 2930 (2008). 22. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, and J.-M. Tarascon, Nature, 407, 496 (2000). 23. C. Suryanarayana, Prog. Mater. Sci., 46, 1 (2001). 24. A. N. Dey, J. Electrochem. Soc., 118, 1547 (1971). 25. L. Y. Beualieu, T. D. Hatchard, A. Bonakdarpour, M. D. Fleischauer, and J. R. Dahn, J. Electrochem. Soc., 150, A679 (2003). 26. S. D. Beattie, T. Hatchard, A. Bonakdarpour, K. C. Hewitt, and J. R. Dahn, J. Electrochem. Soc., 150, A701 (2003). 27. Y. Xu, J. Guo, and C. Wang, J. Mater. Chem., 22, 9562 (2012). 28. M. Marcinek, L. J. Hardwich, T. J. Richardson, X. Song, and R. Kostecki, J. Power Sources, 173, 965 (2007). 29. O. Mao, R. A. Dunlap, and J. R. Dahn, J. Electrochem. Soc., 146, 405 (1999). 30. D. W. Kim, I.-S. Hwang, S. J. Kwon, H.-Y. Kang, K.-S. Park, Y.-J. Choi, K.-J. Choi, and J.-G. Park, Nano Lett., 7, 3041 (2007).