Synthesis of SnO2/Transition Metal Oxide Hollow Nanospheres and Their Electrochemical Properties As Anode for Li-Ion Battery

Lithium-ion batteries (LIBs) are the most popular power source not only for portable electronics but also for upcoming electric vehicles. So far, various materials such as graphitic carbon, Si, Ge, MoO₃, NiO, Fe₂O₃, and SnO₂ have been exploited as the anode materials for LIBs. Among these candidates, SnO₂ is one of the promising materials for anode electrode in LIBs due to its high theoretical capacity (782 mAh g^-1). However, the main difficulties for using the alloy-based materials are their dramatic volume expansion and contraction during Li insertion and extraction. This volume change can generate a large internal stress, leading to pulverization of the electrode and electrical detachment of the active particles. Moreover, formation of Li₂O, related to irreversible capacity, results in a low efficiency and waste of Li⁺. If the conversion reaction of Li₂O, in SnO₂ system, can be induced, the reversible capacity could be increased. Recently, SnO₂-transition metal oxide composites, such as MoO₃, Fe₂O₃, and NiO, etc., have been suggested to induce the synergistic effects between Li₂O, from SnO₂, and transition metal oxide [1-3]. However, most of current studies of these composite materials showed the poor cyclablity and the synergistic effects diminished in the long-term cycling. We believed that these results are realated to the structual instability due to the large volume change during lithium insertion/extraction. Among various nanosturcutres, hollow sphere is the most promising structure because it can accommodate the large volume change and the synthesis process is relatively simple. Thus, we try to synthesize the hollow composite structrue with SnO₂ and transition metal oxides.

In this study, we synthesized multi-layered hollow spheres, composed of SnO₂ and transition metal oxides (Co₃O₄ and Fe₂O₃), by a simple sol-gel based process. Even though cobalt (Co) is well-known catalyst metal for the reversible reaction of Li₂O, it is one of expensive materials. So, we also synthesize the SnO₂@Fe₂O₃ hollow spheres with the similar synthesis procedure. The size of synthesized SnO₂ hollow sphere is about 60 nm and the shell thickness is about 5 nm. After Co₃O₄ and Fe₂O₃ coating, the size was not significantly changed (Fig. 1). The SnO₂@Co₃O₄ and SnO₂@Fe₂O₃@C electrodes showed much higher reversible capacity than SnO₂ hollow sphere electrode (Fig. 2) and further cycling test is also examined. The synergistic effects between SnO₂/Co₃O₄ (Fe₂O₃) and the reaction mechanism observed by TEM and electrochemical analyses will be discussed in this presentation. Furthermore, we also prepared various electrodes such as commercial SnO₂ nanopowder, SnO₂@C hollow spheres, SnO₂@ Fe₂O₃@C solid spheres and the electrochemical performances will be compared. We believe that our studies might suggest the design strategy for the next-generation LIBs, and it can be applied to other SnO₂-metal oxides composites.

Fig. 1 SEM images of (a) SnO₂@Co₃O₄ hollow spheres and (b) SnO₂@Fe₂O₃ hollow spheres.

Fig. 2 Cycling performance of SnO₂, Co₃O₄/SnO_2, and SnO₂@Fe₂O₃@C hollow sphere electrodes.

[1] X. -Y. Xue, Z. -H. Chen, L. -L. Xing, S. Yuan, Y. -J. Chen, Chem. Commun. 47(2011) 5205-5207.

[2] W. Zhou, C. Cheng, J. Liu, Y. Y. Tay, J. Jiang, X. Jia, J. Zhang, H. Gong, H. H. Hng, T. Yu, H. J. Fan, Adv. Funct. Mater. 21(2011) 2439-2445.

[3] M. F. Hassan ,  M. M. Rahman ,  Z. Guo ,  Z. Chen, H. Liu, J. Mater. Chem., 20(2010) 9707-9712.