Amorphized ZnSb-based composite anodes for high-performance Li-ion batteries

Abstract

We report a simple, rapid, and straightforward hybrid mechanochemical synthesis for intermetallic compound-based nanocomposites, which can be used as anode materials for high-performance Li-ion batteries.

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Because of the pending exhaustion of fossil fuels and the environmental problems associated with its use, the search for new materials for energy storage is extremely important for a better future. Among the many energy conversion systems, rechargeable Li-ion batteries are promising for use in hybrid electric vehicles and various portable electronics. Although carbon-based materials, specifically graphite (372 mA h g⁻¹), are currently used as anode materials for Li-ion batteries, many higher-capacity alternatives are being actively pursued.^1–5

Among the many possible alternative anode materials for high-performance Li-ion batteries, Li alloy-based elements, such as Sn, Si, Sb, P, Mg, and Zn have attracted particular attention owing to their ability to reversibly react with large amounts of Li per formula unit.^6–11 Although Li alloy-based systems have a higher energy density, they generally suffer from poor cycle behavior because of the large volume expansion/contraction that occurs during successive discharge/charge processes.

Several materials and methods have been studied intensively and suggested as alternative solutions for the enhancement of the electrochemical performance of Li alloy-based systems, nanomaterials being one of them.^13–15 This is because they offer advantages such as high capacity owing to a high interfacial area, fast rate capability through an increased Li-ion diffusion rate, and high cycling durability by strain accommodation. Despite these benefits, nanosized materials have several disadvantages. For instance, their high surface reactivity can lead to agglomeration during cycling, which results in poor cycling performance. In addition, they are generally synthesized by complex and expensive synthetic routes. Because carbon in the nanocomposite can block the agglomeration of nanomaterials during cycling, its use in synthesizing nanocomposites is suggested as an alternative solution.^{2,3,12,16–18}

The use of intermetallic compounds has also been suggested. The intermetallic compounds can be largely divided into the following two categories when reacted with Li:

Insertion reaction: XY + aLi → Li_aXY (1)

Conversion reaction: XY + aLi → Li_aX + Y (2)

The formation of Li_aXY by the insertion reaction (1) results in good electrochemical behavior thanks to the stable crystalline structures, which is advantageous for high-performance anode materials.^2,10,19–21 On the other hand, the conversion reaction (2) affords nanocrystallites of Li_aX and Y. The nanocrystallites produced by the conversion reaction have an advantageous feature in terms of less severe structural stress.^2,14,22–24 Although various intermetallic compounds have shown enhanced electrochemical properties, the electrochemical performance of these compounds is still poor, thus preventing their commercial use. However, if intermetallic compound-based nanocomposites containing carbon can be synthesized with simpler and less expensive approaches, alloy-based anodes having excellent electrochemical properties can be achieved.

In this study, we aimed to design nanostructured intermetallic compound-based composite anodes with good electrochemical properties such as high initial coulombic efficiency, high energy density, long cyclability, and fast rate capability. A straightforward hybrid mechanochemical reduction method was developed for transforming micron-sized ZnO and Sb₂O₃ powders into new intermetallic ZnSb-based composites modified with carbon.

Fig. 1 shows a schematic diagram for the synthesis of nanostructured ZnSb/M_xO_y (M = Mg or Al)/C composites prepared by the proposed hybrid mechanochemical reduction method. During the process, ZnO and Sb₂O₃ were reduced using Mg or Al as reducing agents, producing ZnSb/M_xO_y (M = Mg or Al)/C composites on the basis of the following thermodynamically feasible reactions. Carbon was added to construct the nanostructured composites.

2ZnO + Sb₂O₃+ 5Mg → 2ZnSb + 5MgO, (ΔG⁰ = −1613.010 kJ mol⁻¹, 298 K) (3)

6ZnO + 3Sb₂O₃+ 10Al → 6ZnSb + 5Al₂O₃, (ΔG⁰ = −4210.116 kJ mol⁻¹, 298 K) (4)

Fig. 1 Schematic diagram for the synthesis of amorphized ZnSb/M_xO_y/C composites by the proposed hybrid mechanochemical reduction process.

We focused on ZnSb/MgO/C composite because it showed better electrochemical performance than the ZnSb/Al₂O₃/C composite. The characterization and electrochemical data of the ZnSb/Al₂O₃/C composite are discussed in the electronic supporting information (ESI, Fig. S1 and S2†).

Fig. 2a and b show the X-ray diffraction (XRD) patterns of the ZnSb and ZnSb/MgO/C composites, respectively, synthesized by the hybrid mechanochemical reduction. All of the peaks of the ZnSb/MgO/C composite corresponded to crystalline MgO, and no other crystalline phases were detected. Fig. 2c shows bright-field transmission electron microscopy (TEM) and HRTEM images, combined with Fourier transformed (FT) patterns of the ZnSb/MgO/C composite. The HRTEM image shows that less than 3 nm-sized amorphized ZnSb and ca. 10 nm-sized nanocrystalline MgO were well dispersed within the amorphous carbon matrix. Additionally, the energy dispersive X-ray spectroscopy (EDS) mapping images confirm that ZnSb, MgO, and carbon were well dispersed within the composite. Therefore, it can be concluded that the ZnSb/MgO/C composite was composed of amorphized (∼3 nm) ZnSb, nanocrystalline (∼10 nm) MgO, and amorphous carbon.

Fig. 2 Characterization of ZnSb/MgO/C composite: (a) XRD patterns of ZnSb and (b) ZnSb/MgO/C, (c) bright-field TEM and HRTEM images with corresponding FT patterns and EDS mapping images of ZnSb/MgO/C.

The voltage profiles of the ZnSb and ZnSb/MgO/C nanocomposite electrodes are shown in Fig. 3a and b, respectively. Pure Zn and Sb electrodes demonstrated very poor electrochemical performances (see ESI, Fig. S3†). The drastic decrease in the capacity of the Zn and Sb electrodes was caused by the large volume change that occurred during formation of the LiZn and Li₃Sb phases, respectively;^25,26 the formation is associated with the pulverization of the active material and its electrical isolation from the current collector. The first discharge and charge capacities of the ZnSb electrode were 684 and 571 mA h g⁻¹, respectively, with a high coulombic efficiency of 83.5%. Although the intermetallic ZnSb electrode showed better electrochemical performance than the Zn and Sb electrodes, the cycling durability was still poor. The synthesized ZnSb/MgO/C composite electrode showed high initial discharge/charge capacities of 645/503 mA h g⁻¹ (ca. 1880/1470 mA h cm⁻³), with a high initial coulombic efficiency of 78.1%. The initial irreversible capacity owing to the ball-milled amorphous carbon content (30 wt%) of the ZnSb/MgO/C composite electrode was approximately 150 mA h g⁻¹, which demonstrates that ZnSb in the composite electrode was fully reversible with Li.²⁷ The full reversibility of ZnSb was attained by the preparation of amorphized ZnSb by the hybrid mechanochemical reduction.

Fig. 3 Electrochemical characteristics of ZnSb and ZnSb/M_xO_y/C composite electrodes: (a) voltage profile of ZnSb (inset: DCP of the first cycle), (b) voltage profile of amorphized ZnSb/MgO/C composite (inset: DCP of the first cycle), (c) cycle performance, and (d) rate capability for the graphite (1 C: 320 mA h g⁻¹), ZnSb, and ZnSb/MgO/C (1 C: 500 mA h g⁻¹) electrodes.

The differential capacity plots (DCP) of the first cycle for the ZnSb and ZnSb/MgO/C composite electrodes are shown in the insets in Fig. 3a and b, respectively. All the peaks are well matched on the same reaction potential, which suggests that these electrodes were involved in three electrochemical reactions. The following results are in good agreement with those of previous ZnSb-related studies:^28,29 (1) the insertion reaction formation of LiZnSb at 0.9 V, (2) the reaction formation of the Li₃Sb phase at 0.67 V, and (3) the LiZn formation reaction at around 0.5–0 V during discharge. The peak broadening near the reaction potentials of the DCP confirmed the low crystallinity of ZnSb within the ZnSb/MgO/C composite. Therefore, the reaction mechanism of the ZnSb/MgO/C composite electrode can be summarized as follows:

During discharge reaction: ZnSb → LiZnSb → Li₃Sb + Zn → Li₃Sb + LiZn (5)

During charge reaction: Li3Sb + LiZn → Li₃Sb + Zn → Li₂ZnSb → Li_2−xZnSb (0 ≤ x ≤ 2) (6)

The ZnSb/MgO/C composite electrode showed dissociation/recombination reactions of (5) and (6) during discharge/charge. Generally, the dissociation reaction of intermetallic X_aY_b (X, Y = Sn, Sb, Zn, P; a, b = 1, 2) compounds into the Li–X and Li–Y phases during discharge and the recombination reaction of X_aY_b nanocrystallites during charge are advantageous in terms of the electrochemical performance of Li alloy-based anodes.^30,31 This is because the intermetallic X_aY_b compounds gradually decreased to 2–3 nm-sized X_aY_b nanocrystallites after a few cycles, and they retained the same size throughout subsequent discharge/charge cycles by repeatedly dissociating into Li–X and Li–Y phases and then recombining to form the intermetallic phase X_aY_b within the composite.³⁰⁻³²

Cycling performance was compared for the ZnSb, ZnSb/Al₂O₃/C, and ZnSb/MgO/C composites and for commercially available mesocarbon microbead (MCMB)-graphite electrodes (current: 100 mA g⁻¹ and voltage range: 0.0–2.0 V). As shown in Fig. 3c, the cycling performance of the ZnSb electrode was very poor as a result of the large volume change that occurred during formation of the LiZn and Li₃Sb phases. The ZnSb/Al₂O₃/C and ZnSb/MgO/C composite electrodes showed high electrochemical reversibility and long cycle behavior as compared with the ZnSb electrode. The ZnSb/MgO/C composite electrode showed a very stable capacity of approximately 443 mA h g⁻¹ (ca. 1300 mA h cm⁻³) over 300 cycles, and its capacity retention after 300 cycles was 88.3% of the first charge capacity. These excellent electrochemical characteristics were attributed to the uniformly distributed, amorphized (∼3 nm) ZnSb, the inactive matrix of nanocrystalline MgO, and the buffering matrix of the amorphous carbon.

The rate capability of the ZnSb/Al₂O₃/C and ZnSb/MgO/C composite electrodes was also tested, and Fig. 3d shows the cycling stability of the ZnSb/Al₂O₃/C and ZnSb/MgO/C nanocomposites as a function of the C rate, with C being defined as the full use of the restricted charge capacity (500 mA h g⁻¹) in 1 h. At rates of 1 C and 3 C, the ZnSb/MgO/C nanocomposite electrode showed very high charge capacities of 465 mA h g⁻¹ and 430 mA h g⁻¹, respectively, corresponding to ca. 90% and 84%, respectively, of the initial charge capacity with stable cycling behavior. The fast rate capability of the amorphized ZnSb/MgO/C composite electrode was attributed to the preparation of the amorphized (∼3 nm) ZnSb by the hybrid mechanochemical synthesis.

Conclusions

We developed a simple, rapid, inexpensive, and scalable method of transforming micron-sized metal oxides into a new amorphized intermetallic compound composite, ZnSb/MgO/C, by a straightforward hybrid mechanochemical reduction method. When tested as an anode material for Li-ion secondary batteries, the amorphized ZnSb/MgO/C composite electrode showed excellent electrochemical performance, including high charge capacities of ca. 503 mA h g⁻¹ or ca. 1470 mA h cm⁻³, a good initial coulombic efficiency of 78.1%, long cyclability with a high capacity retention of 88.3% after 300 cycles, and a fast rate capability with 84% of its initial charge capacity at the 3 C rate. These excellent electrochemical properties of the amorphized ZnSb/MgO/C composite electrode confirm its potential as a new alternative anode material for rechargeable Li-ion batteries. We anticipate that this simple transformation method for the conversion of micron-sized metal oxide powders to intermetallic nanostructured compound composites utilizing the hybrid mechanochemical reduction process will facilitate the use of these composites in many other applications that require uniformly distributed, nanostructured, intermetallic compound composite materials.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2011-0013624).

Notes and references

1. G.-A. Nazri and G. Pistoia, Lithium Batteries: Science and Technology, Kluwer Academic/Plenum, Boston, 2004.
2. C.-M. Park, J.-H. Kim, H. Kim and H.-J. Sohn, Chem. Soc. Rev., 2010, 39, 3115.
3. P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930.
4. R. Marom, S. F. Amalraj, N. Leifer, D. Jacob and D. Aurbach, J. Mater. Chem., 2011, 21, 9938.
5. R. A. Huggins, J. Power Sources, 1999, 81–82, 13.
6. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa and T. Miyasaka, Science, 1997, 276, 1395.
7. C. J. Wen and R. A. Huggins, J. Electrochem. Soc., 1981, 128, 1181.
8. M. Yoshio, T. Tsumura and N. Dimov, J. Power Sources, 2005, 146, 10.
9. A. Netz, R. A. Huggins and W. Weppner, J. Power Sources, 2003, 119–121, 95.
10. D. C. S. Souza, V. Pralong, A. J. Jacobson and L. F. Nazar, Science, 2002, 296, 2012.
11. C.-M. Park and H.-J. Sohn, Electrochim. Acta, 2010, 55, 4987.
12. C.-M. Park and H.-J. Sohn, Adv. Mater., 2007, 19, 2465.
13. A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon and W. V. Schalkwijk, Nat. Mater., 2005, 4, 366.
14. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.-M. Tarascon, Nature, 2000, 407, 496.
15. C. K. Chan, H. Peng, G. Liu, K. Mcilwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31.
16. C.-M. Park and K.-J. Jeon, Chem. Commun., 2011, 47, 2122.
17. M.-S. Park, S. A. Needham, G.-X. Wang, Y.-M. Kang, J.-S. Park, S.-X. Dou and H.-K. Liu, Chem. Mater., 2007, 19, 2406.
18. A. Dailly, J. Ghanbaja, P. Willmann and D. Billaud, Electrochim. Acta, 2003, 48, 977.
19. S. Boyanov, J. Bernardi, E. Bekaert, M. Menetrier, M.-L. Doublet and L. Monconduit, Chem. Mater., 2009, 21, 298.
20. L. M. L. Fransson, J. T. Vaughey, K. Edstrom and M. M. Thackeray, J. Electrochem. Soc., 2003, 150, A86.
21. D. W. Murphy, F. J. Di Salvo, J. N. Carides and J. V. Waszczak, Mater. Res. Bull., 1978, 13, 1395.
22. V. Pralong, D. C. S. Souza, K. T. Leung and L. F. Nazar, Electrochem. Commun., 2002, 4, 516.
23. C.-M. Park and H.-J. Sohn, J. Electrochem. Soc., 2010, 157, A46.
24. C. Kim, M. Noh, M. Choi, J. Cho and B. Park, Chem. Mater., 2005, 17, 3297.
25. Y. Hwa, J. H. Sung, B. Wang, C.-M. Park and H.-J. Sohn, J. Mater. Chem., 2012, 22, 12767.
26. C.-M. Park, S. Yoon, S.-I. Lee, J.-H. Kim, J.-H. Jung and H.-J. Sohn, J. Electrochem. Soc., 2007, 154, A917.
27. J. Sung and C.-M. Park, J. Electroanal. Chem., 2013, 700, 12.
28. C.-M. Park and H.-J. Sohn, Adv. Mater., 2010, 22, 47.
29. S. Saadat, J. Zhu, M. M. Shahjamali, S. Maleksaeedi, Y. Y. Tay, B. Y. Tay, H. H. Hng, J. Ma and Q. Yan, Chem. Commun., 2011, 47, 9849.
30. C.-M. Park and H.-J. Sohn, Chem. Mater., 2008, 20, 6319.
31. L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Nie and J. Liu, Chem. Commun., 2012, 48, 3321.
32. C.-M. Park and H.-J. Sohn, Electrochim. Acta, 2009, 54, 6367.

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Footnote

† Electronic supplementary information (ESI) available: Experimetal procedures, characterization and electrochemical data of the compounds. See DOI: 10.1039/c3ra46609d

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