A nanocomposite of SnO₂ and single-walled carbon nanohorns as a long life and high capacity anode material for lithium ion batteries

Abstract

A novel composite of SnO₂ and single-walled carbon nanohorns (SWCNHs) has been synthesized via a simple wet chemical method. SnO₂ nanoparticles (2–3 nm) were homogeneously distributed on the surface of spherical SWCNHs, as confirmed by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. When evaluated as an anode material for lithium ion batteries, this SnO₂/SWCNHs composite shows superior electrochemical performance with high capacity, excellent cyclic stability and good rate performance, owing to the intimate interaction between the SWCNH matrix and nanosized SnO₂. It delivered a high capacity of 530 mAh g⁻¹ even after 180 cycles under a current density of 500 mA g⁻¹, which is better than most SnO₂ composites.

---

Introduction

Lithium ion batteries (LIBs), as an excellent power source for energy storage devices, have attracted increasing attention in both scientific and industrial fields.¹ As an anode material in LIBs, graphite is commercially used but the theoretical capacity is only 372 mAh g⁻¹. To meet the increasing demand for batteries with higher energy density, transition metal oxides have been intensively investigated as alternative anode materials due to their high capacity.² Among them, SnO₂ is particularly attractive with a theoretical capacity as high as 781 mAh g⁻¹.³ Unfortunately, the aggregation and pulverization of SnO₂ caused by the large volume expansion (>300%) in discharge and charge processes would severely destroy the electrochemical performance. Therefore, nanostructured SnO₂ with various morphologies, including hollow spheres,⁴nanowires,⁵nanotubes,⁶ and nanosheets,⁷ have been designed to solve these problems. For example, Wang et al. synthesized uniform SnO₂ nanoboxes, which demonstrated a specific capacity of 570 mAh g⁻¹ after 40 cycles.⁸ However, nanostructured SnO₂ still suffers from gradual capacity fading and the cycling performance of SnO₂ needs to be improved.

Recently, the composites of SnO₂ and carbonaceous materials have attracted a great deal of attention. Carbonaceous materials, such as carbon coating, single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and graphene nanosheets (GNS) could not only improve the conductivity of SnO₂ composite, but also effectively suppress the large volume expansion of SnO₂ during discharge and charge cycles, thus improving the electrochemical performance of an SnO₂ composite. For example, Lou et al. reported a hollow SnO₂/C composite, which exhibited a capacity of 460 mAh g⁻¹ after 100 cycles at a current density of 500 mA g⁻¹.⁹ Paek et al. synthesized a porous SnO₂/GNS composite, which delivered a capacity of 570 mAh g⁻¹ after 30 cycles at a current density of 50 mA g⁻¹.¹⁰ Wu et al. synthesized MWNTs@SnO₂@C composites which demonstrated a capacity of 462.5 mAh g⁻¹ after 65 cycles at 100 mA g⁻¹.¹¹ However, fabrication of SnO₂ composites with long life and high capacity is still a challenge.

Single-walled carbon nanohorns (SWCNHs), as new carbon nanomaterials, have large surface area, high pore volume and good electric conductivity.^12–20 These properties make it a good carbon matrix to support metal oxides used as anode materials for LIBs.²¹ Here, we prepare a novel SnO₂/SWCNHs composite via a simple wet chemical method. This SnO₂/SWCNHs composite shows high capacity and better cyclic stability than most SnO₂ composites. A capacity of 530 mAh g⁻¹ remained even after 180 discharge and charge cycles under a high current density of 500 mA g⁻¹.

Experimental

Synthesis of SWCNHs

SWCNHs were prepared by DC arc-discharge based on a previous report.²² In brief, pure graphite rods (ϕ = 8 mm) were used as electrodes. An arc was created in air with a pressure of 400 Torr and a current of 120 A. The distance between the two electrodes was kept at about 1 mm. It took 6 min to consume an anode rod. When the discharge ended, the generated soot was collected under ambient conditions. The as-prepared SWCNHs were used without further purification.

Synthesis of SnO₂/SWCNHs composite

In a typical experiment, 0.33 mmol phthalic acid and SnCl₄·5H₂O were dispersed in 50 ml de-ionized water and sonicated for 10 min to form a homogeneous solution. 50 mg SWCNHs was added and the mixtures were sonicated until they dispersed uniformly. Then, 500 mg urea was added into the above dispersion before it was stirred at 80 °C for 20 h. The final products were filtered and washed several times with de-ionized water, then dried at 80 °C overnight. The loading ratio of SnO₂ in SnO₂/SWCNHs composites could be easily controlled by changing the relative amount of Sn⁴⁺ and SWCNHs. Bare SnO₂ nanoparticles were synthesized by the same method without adding SWCNHs.

Sample characterization

The structure and morphology of SnO₂/SWCNHs composite were characterized by X-ray diffraction (XRD, RIGAKU SCXmini), transmission electron microscopy (TEM, JEM-2010), and scanning electron microscopy (SEM, JSM-6700F) equipped with energy dispersive X-ray spectroscopy (EDS). Thermogravimetry analyses (TGA, NETZSCH STA449C) were measured from 30 to 900 °C at a heating rate of 10 K min⁻¹ in air.

Electrochemical measurements

The electrochemical measurements were carried out via a CR2025 coin-type test cell fabricated in a glove box under an argon atmosphere. The working electrode consisted of 80 wt% active material (SnO₂/SWCNHs composite), 10 wt% conductivity agent (ketjen black, KB), and 10 wt% polymer binder (carboxymethyl cellulose, Na–CMC). The electrolyte was 1 M LiPF₆ in EC : EMC : DMC (1 : 1 : 1 in volume). A lithium sheet was used as both the counter and the reference electrode. The electrochemical performance of SWCNHs and bare SnO₂ were also tested under the same method. Cells were discharged and charged on a LAND 2001A system over a range of 0.05 V to 3.00 V at room temperature. Cyclic voltammetry (CV) tests were performed on a CHI660C Electrochemical Workstation with a scan rate of 0.2 mV s⁻¹.

Result and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns of SWCNHs, bare SnO₂ and SnO₂/SWCNHs composite. The major peaks of bare SnO₂ and SnO₂/SWCNHs composite at 26.6°, 33.9°, 51.8°, 64.7° and 65.9° are indexed well to the tetragonal structure of SnO₂ (JCPDS Card No 41-1445). While the additional peak observed at 26° in the SnO₂/SWCNHs pattern is coming from SWCNHs. The broad peaks of pure SnO₂ and SnO₂/SWCNHs composite indicate the small size of SnO₂ particles. The weight percentage of SnO₂ in the SnO₂/SWCNHs composite was determined to be 50 wt% according to the thermogravimetry analysis (Fig. S1, ESI†).

Fig. 1 X-ray diffraction patterns of SWCNHs, bare SnO₂, SnO₂/SWCNHs composite.

The general morphology of SWCNHs and SnO₂/SWCNHs composite were observed by scanning electron microscopy (SEM), shown in Fig. 2. Fig. 2a presents the spherical structure of SWCNHs aggregates with a diameter around 100 nm. After SnO₂ deposition, the obtained SnO₂/SWCNHs still exhibited a spherical morphology, as observed in Fig. 2b. The energy dispersive spectroscopy (EDS) spectrum (Fig. S2, ESI†) clearly revealed this composite consisted of C, Sn, and O elements. In order to identify the distribution of SnO₂ and C, the elemental mapping characterization of the SnO₂/SWCNHs composite was carried out. As can be seen from the element maps of Sn and C in Fig. 2c and d, we could deduce that the SnO₂ nanoparticles were homogeneously distributed on the SWCNH matrix. This morphology of SnO₂/SWCNHs composite could effectively improve the electric connection between SnO₂ nanoparticles and SWCNHs and hinder the aggregation of SnO₂ nanoparticles during discharge and charge cycles.

Fig. 2 SEM images of (a) SWCNHs and (b) SnO₂/SWCNHs; (c) and (d) are the element maps of Sn and C for the SnO₂/SWCNHs composite.

The structure and morphology of SnO₂/SWCNHs composites were further investigated by transmission electron microscopy (TEM). Fig. 3a shows typical ‘dahlia-like’ SWCNHs with SnO₂ nanoparticles homogeneously coated on the surface. Fig. 3b shows the high magnification TEM image of the SnO₂/SWCNHs composite, from which the mean size of SnO₂ nanoparticles can be determined to be about 2–3 nm (more than 200 counts). These ultrafine SnO₂ particles are consistent with the broad peaks in the XRD pattern. The HRTEM image (Fig. 3c) displays SnO₂ nanoparticles strongly anchored on an individual SWCNH, demonstrating the good connection between the SWCNHs and the SnO₂ nanoparticles. The (110) plane of tetragonal SnO₂ can be clearly identified from the interlayer spacing of 0.33 nm. Fig. 3d is the corresponding selected area electron diffraction (SAED) pattern of the SnO₂/SWCNHs composite. The diffraction rings can be ascribed to the (110), (101), (211) diffraction planes of the tetragonal SnO₂ phase. The fabrication of the SnO₂/SWCNHs composite is based on the noncovalent method we have developed to synthesis metal oxides and carbon nanomaterial composites.²³Sn⁴⁺ was firstly complexed with phthalic acid, which was then absorbed on the surface of SWCNHs via noncovalent p–π interaction between phthalic acid and the SWCNHs. When urea was added, OH⁻ generated from its hydrolysis at 80 °C reacted with Sn⁴⁺ and in situ yielded SnO₂ nanoparticles on the surface of the SWCNHs. It is worth noting that the good conductivity of the SWCNHs could be preserved via this noncovalent synthesis method, which is of benefit for the electrochemical performance of the SnO₂/SWCNHs composite.

Fig. 3 TEM images of the SnO₂/SWCNHs composites: (a) low magnification, (b) high magnification, (c) HRTEM and (d) selected area electron diffraction (SAED) pattern.

Cyclic voltammetry (CV) experiments were firstly performed to compare the electrochemical properties of SWCNHs, bare SnO₂ and SnO₂/SWCNHs composite, shown in Fig. 4. Fig. 4a shows typical carbon nanotube-like CVs of SWCNHs.²⁴ There is a strong reduction peak at 0.48 V in the first cathodic scan which disappears in the following cycles. This peak corresponds to the decomposition of electrolyte and formation of the solid electrolyte interface (SEI) film. The reduction and oxidation peaks at 0.05 V and 0.25 V correspond to the insertion and extraction of Li into SWCNHs. Fig. 4b shows the initial three CVs of bare SnO₂ electrode. In the first cathodic scan, the reduction peak at 0.85 V corresponds to the formation of solid electrolyte interface (SEI) layer and the reduction of SnO₂ to metallic Sn, as described in eqn (1) and (2). The other cathodic peak at 0.21 V is ascribed to the formation of a series of Li–Sn alloy. In the anodic scan, the oxidation peak at 0.62 V indicates the highly reversible de-alloying of Li_xSn (eqn (3)). The anodic peak at 1.24 V is ascribed to the partially reversible reaction in eqn (2). While the anodic current at 0.62 V and 1.24 V decreased in the second scan, which would have resulted from the aggregation and pulverization of SnO₂ nanoparticles during the discharge and charge process, suggesting the gradual capacity loss in the following cycles.

Li⁺ + e⁻ + electrolyte → SEI (Li) (1)

4Li⁺ + SnO₂ + 4e⁻ → 2Li₂O + Sn (2)

xLi⁺ + Sn + xe⁻ ↔Li_xSn (0 ≤ x ≤ 4.4) (3)

Fig. 4c shows the CVs of the SnO₂/SWCNHs electrode. There are also two characteristic reduction peaks near 0.89 V and 0.31 V in the first scan, corresponding to the formation of the SEI layer and Li₂O, and formation of the Li_xSn alloy, respectively. The oxidation peaks around 0.58 V and 1.27 V are ascribed to the highly reversible reaction in eqn (3) and partially reversible reaction in eqn (2). However, the CVs of SnO₂/SWNHs display a different electrochemical behavior compared to the bare SnO₂ electrode. The following CVs of SnO₂/SWCNHs electrode kept a similar shape with the first cycle, indicating a better cycle performance of the SnO₂/SWCNHs electrode than bare SnO₂.

Fig. 4 Cyclic voltammograms of (a) SWCNHs, (b) bare SnO₂, and (c) SnO₂/SWCNHs composites at a scan rate of 0.2 mV s⁻¹.

The galvanostatic method was used to estimate the cycling performance of these electrodes between 0.05 V and 3 V. The first discharge and charge curves of SWCNHs, bare SnO₂ and SnO₂/SWCNHs composite are shown in Fig. S3, ESI†. As can be seen from Fig. 5a, SWCNHs delivered first discharge and charge capacities of 732 and 283 mAh g⁻¹. This large capacity loss in the first cycle corresponded to the formation of the SEI layer, which was consistent with the CV observation; while SWCNHs exhibited excellent cycling performance. Even after 100 cycles, the specific capacity of SWCNHs still remained at 240 mAh g⁻¹. As a new carbon anode material for LIBs, SWCNHs have large surface area and excellent cycle performance, which was similar to the other carbon nanostructured materials, such as MWNTs, SWNTs and GNS. Therefore SWCNHs could be a good carbon matrix to support metal oxides and are used as anode materials for LIBs. For a bare SnO₂ electrode, the capacity decreased with the increasing cycle numbers. After 100 cycles, the capacity of SnO₂ was only 65 mAh g⁻¹. The quick capacity fading of bare SnO₂ could as a result of severe aggregation and pulverization of SnO₂ nanoparticles in the charge and discharge processes.

Fig. 5 (a) Cycling performance of bare SnO₂ and SWCNHs at a current density of 150 mA g⁻¹. (b) Cycling performance and coulombic efficiency of the SnO₂/SWCNHs electrode at a current density of 500 mA g⁻¹. (c) Rate capabilities of the SnO₂/SWCNHs composite.

To test the long life cycling performance of SnO₂/SWCNHs composite, the SnO₂/SWCNHs electrode was cycled under a relatively high current density of 500 mA g⁻¹ after being activated at 100 mA g⁻¹ in the first two cycles. As can be seen from Fig. 5b, the specific capacity and cycling performance of the SnO₂/SWCNHs composite was significantly improved compared with bare SnO₂ and SWCNHs. The discharge capacities of the SnO₂/SWCNHs electrode in the 4th, 10th, 50th, 100th, 150th cycle were 710, 649, 595, 567 and 537 mAh g⁻¹, respectively, indicating the high capacity and good cycle stability of the SnO₂/SWCNHs composite. Even after 180 discharge and charge cycles, the specific capacity of SnO₂/SWCNHs electrode still remained at 530 mAh g⁻¹, which is much higher than the theoretical capacity of commercial graphite of 372 mAh g⁻¹. The average capacity loss was only 0.2% per cycle during the 180 cycles. Meanwhile, the coulombic efficiency of the SnO₂/SWCNHs electrode after the first five cycles kept around 98% until 180 cycles. The electrochemical performance of SnO₂/SWCNHs composite (530 mAh g⁻¹ after 180 cycles at 500 mA g⁻¹) is better than nanostructured SnO₂ with various morphology,^7–8,25SnO₂/C composites,^9,26SnO₂/MWNTs composites^11,27–28 and SnO₂/GNS composites.^10,29 We also synthesized other SnO₂/SWCNHs composites with higher SnO₂ content (such as 60 wt% SnO₂). The TGA and electrochemical performance of (60% SnO₂)/SWCNHs composite are shown in Fig. S4 and S5, ESI†. The specific capacity of (60% SnO₂)/SWCNHs delivered a high capacity of 700 mAh g⁻¹ after 30 cycles. While it suffered from gradual capacity fading. After 180 cycles, the capacity was only 330 mAh g⁻¹. This capacity fading phenomenon was due to the excess SnO₂ nanoparticles, which would not be coated on the surface of SWCNHs and aggregated together outside the SWCNHs if we increased the loading ratio of SnO₂ in this composite. So a loading ratio of 50% is sufficient for this composite to be used as an anode material to achieve high capacity and good cycling performance.

Fig. 5c exhibits the rate performance of the SnO₂/SWCNHs composite under various current densities from 100 to 1500 mA g⁻¹. The discharge and charge profiles of the SnO₂/SWCNHs electrode under different current densities are shown in Fig. S6, ESI†. These curves kept a similar shape and delivered high capacities, demonstrating the excellent rate capabilities of the SnO₂/SWCNHs composite. The specific capacities of the SnO₂/SWCNHs composite were 670, 610, 530 mAh g⁻¹ when cycled at 300, 500 and 1000 mA g⁻¹, respectively. Even under a high current density of 1500 mA g⁻¹, the specific capacity remained at ∼390 mAh g⁻¹, still higher than commercial graphite anode (~372 mAh g⁻¹). Furthermore, a stable discharge capacity of 700 mAh g⁻¹ could be recovered when the current density was reduced back to 100 mA g⁻¹, indicating the good cycle stability of the SnO₂/SWCNHs electrode. Commonly, eqn (3) is thought to be highly reversible, giving a theoretical capacity of 781 mAh g⁻¹ for SnO₂. The SnO₂/SWCNHs composite consists of 50 wt% SnO₂ and 50 wt% SWCNHs. The theoretical capacity of SnO₂/SWCNHs could be calculated as follows: C_theoretical = C_SnO2 × %mass of SnO₂ + C_SWCNHs × %mass of SWCNHs = 781 × 50% + 240 × 50% = 511 mAh g⁻¹. However, the measured capacity (700 mAh g⁻¹) is higher than the theoretical capacity of SnO₂/SWCNHs. We deduce the excess capacity is coming from the partially reversible reaction of eqn (2), which is consistent with the CV observation of the SnO₂/SWCNHs composite. The high capacity, excellent cyclic stability and good rate performance of the SnO₂/SWCNHs composite could be attributed to the intimate interaction between the SnO₂ nanoparticles and the SWCNHs. The SWCNHs can improve the conductivity of this composite and hamper the aggregation of SnO₂ nanoparticles to alleviate the degrading of the electrode. Meanwhile SnO₂ nanoparticles (2–3 nm) which were distributed well on the SWCNHs could provide a large electrode/electrolyte contact area and short path length for Li⁺ transport, thus improving the rate performance.

Conclusion

In summary, we successfully synthesized a novel SnO₂/SWCNHs composite via a simple wet chemical method. As an anode material for lithium ion batteries, the SnO₂/SWCNHs composite exhibits superior electrochemical performance with high capacity, excellent cyclic stability and good rate performance, highlighting the importance of the intimate interaction between SWCNHs substrate and SnO₂ nanoparticles. This SnO₂/SWCNHs composite delivers a high capacity of 530 mAh g⁻¹ even after 180 cycles at a current density of 500 mA g⁻¹, which make it a promising anode material for LIBs.

Acknowledgements

We acknowledge the financial support provided by the National Key Project on Basic Research (grant no. 2009CB939801, 2011CB935904), Natural Science Foundation of Fujian Province (Grant No. 2010J05041), and Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (Grant No. SZD09003).

References

1. L. Ji, Z. Lin, M. Alcoutlabi and X. Zhang, Energy Environ. Sci., 2011, 4, 2682–2699.
2. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496–499.
3. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa and T. Miyasaka, Science, 1997, 276, 1395–1397.
4. S. J. Han, B. C. Jang, T. Kim, S. M. Oh and T. Hyeon, Adv. Funct. Mater., 2005, 15, 1845–1850.
5. M. S. Park, G. X. Wang, Y. M. Kang, D. Wexler, S. X. Dou and H. K. Liu, Angew. Chem., Int. Ed., 2007, 46, 750–753.
6. Y. Wang, M. H. Wu, Z. Jiao and J. Y. Lee, Nanotechnology, 2009, 20, 7.
7. C. Wang, Y. Zhou, M. Y. Ge, X. B. Xu, Z. L. Zhang and J. Z. Jiang, J. Am. Chem. Soc., 2010, 132, 46–47.
8. Z. Y. Wang, D. Y. Luan, F. Y. C. Boey and X. W. Lou, J. Am. Chem. Soc., 2011, 133, 4738–4741.
9. X. W. Lou, C. M. Li and L. A. Archer, Adv. Mater., 2009, 21, 2536–2539.
10. S. M. Paek, E. Yoo and I. Honma, Nano Lett., 2009, 9, 72–75.
11. P. Wu, N. Du, H. Zhang, J. X. Yu and D. R. Yang, J. Phys. Chem. C, 2010, 114, 22535–22538.
12. S. Iijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai and K. Takahashi, Chem. Phys. Lett., 1999, 309, 165–170.
13. K. Murata, K. Kaneko, W. A. Steele, F. Kokai, K. Takahashi, D. Kasuya, K. Hirahara, M. Yudasaka and S. Iijima, J. Phys. Chem. B, 2001, 105, 10210–10216.
14. S. Utsumi, J. Miyawaki, H. Tanaka, Y. Hattori, T. Itoi, N. Ichikuni, H. Kanoh, M. Yudasaka, S. Iijima and K. Kaneko, J. Phys. Chem. B, 2005, 109, 14319–14324.
15. E. Bekyarova, K. Kaneko, M. Yudasaka, K. Murata, D. Kasuya and S. Iijima, Adv. Mater., 2002, 14, 973–975.
16. C. M. Yang, Y. J. Kim, M. Endo, H. Kanoh, M. Yudasaka, S. Iijima and K. Kaneko, J. Am. Chem. Soc., 2007, 129, 20–21.
17. K. Urita, S. Seki, S. Utsumi, D. Noguchi, H. Kanoh, H. Tanaka, Y. Hattori, Y. Ochiai, N. Aoki, M. Yudasaka, S. Iijima and K. Kaneko, Nano Lett., 2006, 6, 1325–1328.
18. Y. Tao, D. Noguchi, C. M. Yang, H. Kanoh, H. Tanaka, M. Yudasaka, S. Iijima and K. Kaneko, Langmuir, 2007, 23, 9155–9157.
19. A. Izadi-Najafabadi, T. Yamada, D. N. Futaba, M. Yudasaka, H. Takagi, H. Hatori, S. Iijima and K. Hata, ACS Nano, 2011, 5, 811–819.
20. S. Y. Zhu and G. B. Xu, Nanoscale, 2010, 2, 2538–2549.
21. Y. Zhao, J. X. Li, Y. H. Ding and L. H. Guan, Chem. Commun., 2011, 47, 7416–7418.
22. N. Li, Z. Y. Wang, K. K. Zhao, Z. J. Shi, Z. N. Gu and S. K. Xu, Carbon, 2010, 48, 1580–1585.
23. Y. Zhao, J. X. Li, C. X. Wu and L. H. Guan, Nanoscale Res. Lett., 2011, 6, 71–75.
24. J. X. Li, C. X. Wu and L. H. Guan, J. Phys. Chem. C, 2009, 113, 18431–18435.
25. J. F. Ye, H. J. Zhang, R. Yang, X. G. Li and L. M. Qi, Small, 2010, 6, 296–306.
26. B. H. Zhang, X. Y. Yu, C. Y. Ge, X. M. Dong, Y. P. Fang, Z. S. Li and H. Q. Wang, Chem. Commun., 2010, 46, 9188–9190.
27. Z. H. Wen, Q. Wang, Q. Zhang and J. H. Li, Adv. Funct. Mater., 2007, 17, 2772–2778.
28. L. Noerochim, J. Z. Wang, S. L. Chou, H. J. Li and H. K. Liu, Electrochim. Acta, 2010, 56, 314–320.
29. M. Zhang, D. Lei, Z. F. Du, X. M. Yin, L. B. Chen, Q. H. Li, Y. G. Wang and T. H. Wang, J. Mater. Chem., 2011, 21, 1673–1676.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1ra00267h

---